Users with multiple computers should be aware that Library Sharing between iTunes 7 and previous versions is not enabled. I have three primary computers in my home; two running OS X Tiger and one running Windows XP. My main music library is on a USB hard drive connected to my desktop computer, and on my laptop and Windows computer I normally listen to music via shared libraries, streamed over my wireless connection.
I upgraded to iTunes 7 on my primary desktop, and have discovered that computers [both Windows and Mac] running iTunes 6 [and I assume lower] can no longer connect to the iTunes 7 computer's shared library, with a "Shared Music Library Name is not compatible with this version of iTunes." error message.
However, iTunes 7 is still able to connect to shared libraries on both Windows and Mac iTunes 6 clients -- so if you're an early-adopter and listen to friend's or co-worker's music, you don't have much to worry about ... but if they like to listen to your music, you should consider staying with iTunes 6 or encouraging them to upgrade to iTunes 7.