Cross-sectional lifespan studies do show decreasing numbers of left-handers as the population ages, which does suggest that the sinister types are dying off at a faster rate than their dexterous companions. Assorted speculations exist as to why: the subtle effects on accident rates of living in a world structured on the assumption of right-handedness, the possible connection between perinatal stress and left-handedness, etc.
However, there is one significant confound with this finding: some cultures have a recently-abandoned prejudice against left-handedness. Due to this historical bias, many older individuals were raised in circumstances that actively encouraged left-handed individuals to adapt to right-handedness. The gradual dissipation of this trend (see Barsley M (1970) Left-handed man in a right-handed world. Pitman London. & Corballis MC (1993) The lopsided ape: Evolution of the generative mind. Oxford University Press, USA. p89. for discussion of the decline in handedness-related prejudice) could produce an increase in the number of detectable left-handers as time progresses without any need for differences in mortality.
See Coren S, Halpern DF (1991) Left-handedness: A marker for decreased survival fitness. Psychological bulletin 109:90-106 and Harris LJ (1993) Do left-handers die sooner than right-handers? Commentary on Coren and Halpern's (1991) "Left-handedness: A marker for decreased survival fitness". Psychological Bulletin 114:203-234 for arguments supporting both sides of this issue.
For a lengthy (and entertaining) discussion on the nature of handedness (and its possible connections to lateralization of brain function and the evolution of language) which includes a review of the evidence relating to left-handed survival rates, see Michael Corballis’ books “The Lopsided Ape” and “From Hand to Mouth”. Corballis MC (1980) Laterality and myth. American Psychologist 35:284 is worth a look as well.