Any answer with fewer than 2 upvotes will not be auto-accepted; you have to manually accept it. This is part of the "contract," if you will, that starting a bounty gives you -- you're not supposed to let it run out, you're supposed to select an answer. This is by design.
Rules of bounties:
All of this can also be found in the FAQ:
Once initiated, the bounty period
lasts 7 days. If you mark an accepted
answer, your bounty is awarded to the
answerer (do note that accepted bounty
answers are permanent and cannot be
changed). If you do not accept an
answer in 7 days, here is what happens
at the end of the bounty period:
- The highest voted answer created after the bounty started with at least
2 upvotes will be automatically
accepted. Half the bounty will be
awarded to the owner of that answer.
- If there is no answer meeting the above criteria, and you as the
question owner have not opted to
accept an answer, you forfeit your
right to ever accept an answer to your
question.
In any case, you will always give up
the amount of reputation specified in
the bounty, so if you start a bounty,
be sure to follow up and accept the
best answer!