I suspect that no definitive answer is known, but below is some interesting research. In general, it sounds like little research has been done to directly test the question humans. Epidemiological evidence suggest otherwise, though that may be not be causation (ie overweight people trying to stem weight gain might tend to eat artificial sweeteners more). If the food reward hypothesis of obesity is correct, that would also suggest that artificial sweeteners wouldn't necessarily aid in weight loss if they increase the palatability of caloric food.
A recent small study in rats suggested artificial sweeteners can increase obesity, though nutritionist Marion Nestle is not particularly convinced.
The most informative reference I can find (other than the paper mentioned by @Oliver_C in the comments) is this paper Gain weight by “going diet?” Artificial sweeteners and the neurobiology of sugar cravings. From the weight loss section of the paper:
do artificial sweeteners actually help reduce weight?
Surprisingly, epidemiologic data suggest the contrary. Several large scale prospective cohort studies found positive correlation between artificial sweetener use and weight gain. The San Antonio Heart Study examined 3,682 adults over a seven- to eight-year period in the 1980s. When matched for initial body mass index (BMI), gender, ethnicity, and diet, drinkers of artificially sweetened beverages consistently had higher BMIs at the follow-up, with dose dependence on the amount of consumption. Average BMI gain was +1.01 kg/m2 for control and 1.78 kg/m2 for people in the third quartile for artificially sweetened beverage consumption. The American Cancer Society study conducted in early 1980s included 78,694 women who were highly homogenous with regard to age, ethnicity, socioeconomic status, and lack of preexisting conditions. At one-year follow-up, 2.7 percent to 7.1 percent more regular artificial sweetener users gained weight compared to non-users matched by initial weight. The difference in the amount gained between the two groups was less than two pounds, albeit statistically significant. Saccharin use was also associated with eight-year weight gain in 31,940 women from the Nurses’ Health Study conducted in the 1970s.
Similar observations have been reported in children. [...] In addition, consensus from interventional studies suggests that artificial sweeteners do not help reduce weight when used alone. BMI did not decrease after 25 weeks of substituting diet beverages for sugar-sweetened beverages in 103 adolescents in a randomized controlled trial, except among the heaviest participants. A double blind study subjected 55 overweight youth to 13 weeks of a 1,000 Kcal diet accompanied by daily capsules of aspartame or lactose placebo. Both groups lost weight, and the difference was not significant. Weight loss was attributed to caloric restriction. Similar results were reported for a 12-week, 1,500 Kcal program using either regular or diet soda. Interestingly, when sugar was covertly switched to aspartame in a metabolic ward, a 25 percent immediate reduction in energy intake was achieved. Conversely, knowingly ingesting aspartame was associated with increased overall energy intake, suggesting overcompensation for the expected caloric reduction. Vigilant monitoring, caloric restriction, and exercise were likely involved in the weight loss seen in multidisciplinary programs that included artificial sweeteners.
Edit: July 7, 2013: This opinion/summary piece from in Cell gives a good summary. Based on the research discussed in the paper, the answer to your original question is: the best available evidence points to yes, artificial sweeteners can make you gain weight.