Xcode 4 and iTunes
For others who may experience a similar issue
I'd just got my iPad 2 and connected it up to my Mac machine running the latest Mac OS X v 10.6.7 and started iTunes as instructed.
Things seemed to be going fine through registration and setup when I received the abrupt warning
iTunes was unable to load dataclass information from Sync Services. Reconnect or try again later.
iTunes did, however, recognise that an upgrade to iOS 4.3.3 was required and proceeded with the download.
At the start of the iOS upgrade, however, I got the equally strange message
iTunes could not backup KML System's iPad because the backup session failed.
Tried all the usual things, checked iTunes version was up-to-date, rebooted machine, etc. etc.
Was there a problem because I had Dropbox running ? (this didn't seem to be a problem)
How about trying the 'Restore to original settings' ? (This did proceed to install iOS 4.3.3 on the iPad but the same messages persisted and sync would still not work).
Was there something wrong with the iPad ? (brand new too! - but no, the same problem was apparent with my 2nd Generation iPod Touch which produced the same messages when plugged in).
To cut a long story short, it appears that installing Xcode 4 will corrupt an existing installation of iTunes in some way. The solution is simply to reinstall iTunes!!
Lo and behold, the problem went away after a re-install of iTunes and all is back in working order with syncing and backups working as per normal.
ps I came across an interesting topic on the Apple Support Communities which discusses the same issue and contains some additional steps which may be required if a simple re-install of iTunes fails to resolve the issue.