iTunes download version 10(www.itunes.com/download) was released just moment ago, but there cannot the iTunes 10 sync music with iPhone.
To fix iTunes download version 10 cannot sync music with iPhone 3Gs, you needn’t to downgrade your iTunes 10 to iTunes 9.
You need to do is just as follows:
In the transition from iTunes 9 to iTunes 10, all your music gets unchecked.
#1. it has to be checked with www.itunes.com
#2. Go to the top of your Library to “Music”. Make sure you are in that directory.
#3. Go to to “Edit” (usually at the top of your screen) then “Select All”.
#4. Right click your mouse then “Check Selection”.
#1. Go to your iPhone tab in iTunes 10, and then go to Music, Movie, etc.
#2. Recheck the them and apply www.itunes.com/sync in iTunes download version 10.
This will check your entire music library. Then you can sync your iPhone, iPod to iTunes download version 10.