I have the same problem today. I upgraded from 10.4 to 10.5 during the week, this is the first time I've hooked up the iPhone to it, its charging but won't sync or even appear in iTunes. I haven't disabled automatic synching, I have restarted the iPhone and the computer and switched USB ports.
Its a brand new phone this week, I had it replaced at the store (turns out it was only the headphones but they replaced the whole phone first and I had to go back and get replacement headphones), I restored it from backup on this computer (MacBook) on Wednesday and bought and installed 10.5 on Thursday. The backups are still in the iTunes synching preferences. It appears on my Mac Pro at work (I don't sync it there, just charge it on that computer, but it appears in iTunes on that, and thats 10.4).