Dear Marcy,
Do not worry about your vote bringing Obama into office. The Zogby poll has California at Obama 52%, McCain 32%. So, Obama will carry California with or without you. If you vote for Obama or McCain, your vote will get lost amongst the 100’s of thousands of votes they get in California, and will make no difference in the result or even the perception of the result.
A write in vote for Ron Paul will most likely not even be counted. If, hypothetically, votes for Ron Paul are counted, and he gets close to or more votes than Barr, you will have helped kill the Libertarian Party, and in its place established a cult of personality. When Paul is gone, it will be the same as with Ross Perot - there will nothing lasting left to show for the effort.
If you want to cast your vote for Liberty, then you should vote for Bob Barr, and help build the LP into a lasting bastion of liberty. Only large LP vote totals will influence the powers that be. As far as they are concerned, Paul has already been neutralized - ever since he dropped out of the Republican primary. Only Barr and the threat of a growing LP can make an impact, now.
Rich