I'm surprised that people who wanted this feature haven't already enabled manual-sync. Is there really some benefit in having it automatically sync your device to be worth the pain of not being able to plug it in somewhere else?
I went a step further and edited the iTunes DB file's device ids so that they are the same across all 3 of my systems. So my iPhone now recognizes each as the same system allowing me to sync my music with one of my laptops and my videos with my desktop.
The downsides which are quite irritating depending on your listening habits are:
1. Having to manually manage all songs, which gets tiring if you constantly add new music to the library
2. None of the listening history and ratings are synced over when connecting a manually managed device. This ties in with #1: when listening to a bunch of new albums, it's helpful to preserve the ratings to make knowing what albums and tracks to trash easier later on.
I could never get it to work with my iPhone, even with "manual sync". There is the possibility I was doing something wrong, but with iTunes 10 it doesn't matter at this point.
I went a step further and edited the iTunes DB file's device ids so that they are the same across all 3 of my systems. So my iPhone now recognizes each as the same system allowing me to sync my music with one of my laptops and my videos with my desktop.