This is one I've meant to look at for a while.
The Republicans have on more than one occasion mocked Senator Obama for voting "present" rather than yes or no when he was a state senator. The most complete analysis of this that I have found is here, with sources dating back to last December.
The key facts are these:
a) Barack Obama voted "present" 129 times out of "roughly 4,000" votes, so about 3 percent of the time.
b) In the Illinois legislature, voting "present" allows individuals or parties to express opposition to a measure without taking the political fallout of voting no.
c) Sometimes Barack Obama voted "present" according to party dictate and sometimes as an individual.
The main thing I take away from this is that voting "present" is a political option often undertaken for political reasons and is not merely or necessarily an absence of nerve or opinion.