The CDC website has a nice summary of our current knowledge on the matter.
Dangers of Mixing Alcohol and Energy Drinks
Energy drinks are beverages that typically contain caffeine, other plant-based stimulants, simple sugars, and other additives. They are very popular among youth and are regularly consumed by 31% of 12- to 17-year-olds and 34% of 18- to 24-year-olds.
When alcoholic beverages are mixed with energy drinks, a popular practice among youth, the caffeine in these drinks can mask the depressant effects of alcohol. At the same time, caffeine has no effect on the metabolism of alcohol by the liver and thus does not reduce breath alcohol concentrations or reduce the risk of alcohol-attributable harms.
Drinkers who consume alcohol mixed with energy drinks are 3 times more likely to binge drink (based on breath alcohol levels) than drinkers who do not report mixing alcohol with energy drinks.
Drinkers who consume alcohol with energy drinks are about twice as likely as drinkers who do not report mixing alcohol with energy drinks to report being taken advantage of sexually, to report taking advantage of someone else sexually, and to report riding with a driver who was under the influence of alcohol.
There is a much more in depth review available. I've read most of the linked articles and this seem to me to be a comprehensive and adequate review of what has been published up to 2008.
Drug Alcohol Depend. 2009 Jan 1;99(1-3):1-10. doi: 10.1016/j.drugalcdep.2008.08.001. Epub 2008 Sep 21.
Caffeinated energy drinks--a growing problem.
Reissig CJ, Strain EC, Griffiths RR.
There is an association between the heavy use of caffeine and the heavy use of alcohol (Istvan and Matarazzo, 19841; Kozlowski et al., 19932), and the ingestion of energy drinks in combination with alcohol is becoming increasingly popular (O’Brien et al., 20083; Oteri et al., 20074), with 24% of a large stratified sample of college students reporting such consumption within the past 30 days (O’Brien et al., 20083). In the previously mentioned survey of 496 college students, 27% reported mixing alcohol and energy drinks in the past month. Of those that mixed energy drinks and alcohol, 49% used more than three energy drinks per occasion when doing so (Malinauskas et al., 20075). In a survey of 1,253 college students, energy drink users were disproportionately male and consumed alcohol more frequently than non-energy drink users (Arria et al., 20086).
One study showed that ingestion of a caffeinated energy drink (Red Bull) with vodka reduced participants perception of impairment of motor coordination in comparison to vodka alone, but did not significantly reduce objective measures of alcohol-induced impairment of motor coordination, reaction time, or breath alcohol concentration (Ferreira et al., 20067). These results are consistent with other studies investigating caffeine-alcohol interactions (Marczinski and Fillmore, 20068). Thus, when mixing energy drinks and alcohol, users may not feel the symptoms of alcohol intoxication. This may increase the potential for alcohol-related injury. Indeed, a recent survey of college students found that in comparison to those who consumed alcohol alone, students who consumed alcohol mixed with energy drinks had a significantly higher prevalence of alcohol-related consequences including: being taken advantage of, or taking advantage of another student sexually, riding in an automobile with a driver under the influence of alcohol, or being hurt or injured (O’Brien et al., 20083). In addition, mixing energy drinks with alcohol was associated with increased heavy episodic drinking and episodes of weekly drunkenness (O’Brien et al., 20083). The recent introduction of pre-mixed caffeine-alcohol combination drinks may exacerbate these problems (Simon and Mosher, 20079) and has prompted regulatory action. Accordingly, as part of a legal settlement reached in 2008 with State Attorneys in eleven states in the U.S., Anheuser-Busch has agreed to stop the manufacture and sale of caffeinated alcoholic beverages (Idaho Office of the Attorney General website, 200810).
1 Istvan J, Matarazzo JD. Tobacco, alcohol, and caffeine use: a review of their interrelationships. Psychol Bull. 1984;95:301–326.
2 Patterns of alcohol, cigarette, and caffeine and other drug use in two drug abusing populations.
Kozlowski LT, Henningfield JE, Keenan RM, Lei H, Leigh G, Jelinek LC, Pope MA, Haertzen CA
J Subst Abuse Treat. 1993 Mar-Apr; 10(2):171-9.
3 Caffeinated cocktails: energy drink consumption, high-risk drinking, and alcohol-related consequences among college students.
O'Brien MC, McCoy TP, Rhodes SD, Wagoner A, Wolfson M
Acad Emerg Med. 2008 May; 15(5):453-60.
4 Intake of energy drinks in association with alcoholic beverages in a cohort of students of the School of Medicine of the University of Messina.
Oteri A, Salvo F, Caputi AP, Calapai G
Alcohol Clin Exp Res. 2007 Oct; 31(10):1677-80.
5 A survey of energy drink consumption patterns among college students.
Malinauskas BM, Aeby VG, Overton RF, Carpenter-Aeby T, Barber-Heidal K
Nutr J. 2007 Oct 31; 6():35.
6 Arria AM, Caldeira KM, O’Grady KE, Vincent KB, Griffiths RR, Wish ED. Energy drink use is associated with subsequent nonmedical prescription stimulant use among college students.
Note: I haven't found the previous article on the corresponding team website.
7 Effects of energy drink ingestion on alcohol intoxication.
Ferreira SE, de Mello MT, Pompéia S, de Souza-Formigoni ML
Alcohol Clin Exp Res. 2006 Apr; 30(4):598-605.
8 Clubgoers and their trendy cocktails: implications of mixing caffeine into alcohol on information processing and subjective reports of intoxication.
Marczinski CA, Fillmore MT
Exp Clin Psychopharmacol. 2006 Nov; 14(4):450-8.
9 Simon M, Mosher J. Alcohol, energy drinks, and youth: A dangerous mix. Marin Institute; San Rafael CA: 2007.
10 State of Idaho Office of Attorney General Lawrence Wasden website. 2008.