My iTunes was already on for cellular data... but real point - as per a few posts up - you don't need to do any fix. Just go straight to the apps you use or the to the iTunes store an download any apps you want that support this new feature.
The error, you'd think, suggests that you have to download special plug ins or special Passbook apps... not the case at all.
Apps with passbook support are no different than any normal app. This is a feature they support. i already had the AA app installed and was surprised that it magically worked. i was trying to to get passbook 'to work' - basically gave up when the tricks to get iTunes to connect didn't work.. went to the AA app I normally use for my boarding pass and the option to put my pass in Passbook was there. Clicked to add -and VOILA!
It's annoying this error is happening... and it confuses the expectation of how passbook actually works... Bottom line is can completely bypass the non-issue entirely.