Two things come to mind. First, a lot of it comes down to the way in which many celebrities get involved in politics. The problem with the Dixie Chicks, Sarandon, Streisand, etc is that they just use their celebrity as a bully puplit without ever actually doing anything (I know some celebs are actively involved with various political organizations, but the vast majority of them do nothing but throw money at interest groups and talk out their asses to the press). And when they do talk, a lot of what comes out are negative remarks, snide comments, and lots of complaining without any proposal of a well thought-out solution. "I hate Bush" isn't profound political commentary; "I think we need to reinvestigate the role of the UN in Iraqi reconstruction and offer such concessions as... to get them to assist us" is. Unfortunately most of the stuff that comes out is the former, and there's understandable backlash in the public because it just sounds whiny and irritating without anything constructive to offer. When you say nothing but negatives, people tend to percieve you as an ass and avoid you. And please, don't start that nonsense about how public backlash against political remarks constitutes some sort of oppression of free speech. When your career is based upon your public image, you have to avoid alienating a good percentage of the population if you want to stay successful. Celebs can still speak out with whatever political opinion they wish, and the public has the same right to express disagreement, criticize the celebrity, and boycott his products in response.
Second, Arnold is somewhat of an exception because he put his money where his mouth was and ran for office. As another poster said, this was his own doing, not the result of political maneuvering. Arnold also had somewhat of a past reputation for his interest in politics, so his candidacy did not just come out of nowhere, as it did with the Dixie Chicks. The main reason many Republicans backed him was because his celebrity (and moderate views) made him a much more electable candidate than any conservative, and it makes sense to support the candidate who you think has the best shot of winning, provided that his views generally coincide with your own. Another thing that attracts people about Arnold is that, save the last minute mudslinging with Davis in the campaign, his political comments have been very positive and he is starting to make efforts to win the support of Democrats rather than attacking them as the enemy. Granted he has been vague, but instead of just trashing people who don't share his opinion, he has taken a very optimistic tone in his speeches, and that's a lot more attractive to the public than the antagonistic way many other celebrities have acted in voicing their political dissent.
On another note, please cut out all the ad hominems and insulting remarks about anyone who disagrees with you. It's hard to give you any creedence when every other sentence is some nasty remark about conservatives or Bush or whomever. It doesn't make your argument any stronger; on the contrary it makes you look like you're trying to fill holes in your logic by throwing out insults, and just looks immature. Are you trying to win people over to your side or just irritate people? It clearly seems like the latter, because if you wanted to get people who disagree with you to change their minds, you'd actually be civil instead of just offensive.