Tony's Tips ~ Syncing with Multiple Computers
As it turns out Apple and iTunes is more prepared for this than it appears, they just don't advertise or explain it very well to anyone. An iPhone isn't tied to just one machine for syncing, it's simply tied to one machine for syncing each category. In other words you could have your music sync from one machine, photos from another, movies from a third machine, etc. for each separate tab in the iTunes menu. So if you click the "Sync Movies" check box and it warns you it will erase all Movies on your phone you can proceed without fear of losing all of your music as long as the music check box is not marked as well.
Go forth my friends and buy as many computers as you can afford, iPhone syncing can no longer hold you back from your dream of having an AT&T-like command center in your basement.