Heres a strange situation. I have used iPods in the past and just got my first iPhone recently. With the iPods I always managed my music manually in iTunes as opposed to auto syncing. I often connected my iPod to two different computers and it worked fine. I could access the iPod on either computer and add or delete music (from either computer). I set my new iPhone 3g up on my laptop and used it there without issue. It too is set for managing music manually. The laptop is Windows Vista. I tried connecting the iPhone to my Windows XP desktop like I did my iPod and it wouldnt let me (iTunes gave me a dialogue box that said it would erase the music library on the iPhone. In fact I tried this and sure enough it wiped out my music). I was about to give up and just use it on my laptop only when I thought Id try a crazy idea. I went to the My Music file on my desktop and deleted the iTunes folder. I then went to my laptop and copied the iTunes folder from the Music folder and pasted it to the My Music folder on my desktop where the one I had previously deleted resided. Basically I replaced the iTunes folder on my desktop with the one that worked from my laptop. One key thing to consider was that I had no music or videos on the desktop at the time. If I had music there the deleting the iTunes folder would have hosed it. Of course when I went into iTunes after copying the folder from the laptop it listed a bunch of songs from my laptop that werent really on my desktop. So I just deleted them (the listings in iTunes; the files werent really there anyway). Believe it or not this worked. Now when I connect the iPhone to my laptop or desktop it behaves like my iPod did; it is accessible in manual mode on both machines. I dont know if others have had a similar problem but I thought I would pass this along just in case.