Ok, I was able to get my iPhone syncing with multiple libraries on iTunes 10, with two Intel iMacs (home & work). Long story short, I just replaced the ENTIRE iTunes Music Library.xml file and the "iTunes Library" file.
Like everyone, I tried editing the iTunes Music Library.xml file and the iTunes Library file, changing the persistent ID, to match with my home iMac, but no luck. So I wondered what would happen if I just trashed both of those files on the second Mac and replaced them with the two files from home? It works! When I started up iTunes on the second Mac, the "Manually manage music" box was already checked under my iPhone preference tab.
But if you do this, upon starting up iTunes after the swap, you will see your first computer's music listed and most of it will not be found of course. So the thing to do is to "select all" songs in iTunes, hit DELETE, and it will ask if you want to also move the original files to trash or keep them? Choose "keep songs" because you might have some in there that are the same as from home. Now open your actual iTunes music folder on this second computer, do "select all" on the album folders and drag them all into the iTunes left column at top, then let go. Give your Mac a few minutes to process the songs and add them back to its database. This might seem to freeze up your Mac for a few seconds but it will soon start churning.
PLEASE back up your iTunes data files and all your music before trying this! Disclaimer: doing this might cause you to lose playlists you already made on the second Mac, and maybe other things. My only goal in this was to be able to add songs to my iPhone from a second Mac, so I can only speak for that. And I don't use Windows so I can't help you there.