My guess is that iTunes 8 will be powered by Quicktime 8 and that is the delay.
It's a sort of educated guess, we haven't had an iTunes bump in a while and we haven't had a Quicktime bump in an even longer while. Quicktime needs some major work as it has security problems due to legacy code. Quicktime is way too important to get wrong as it underpins much of Apple's success (iTunes, iLife, OS X etc).
I wouldn't rule it out though, although it is more likely to arrive at the iPod event, simply because Apple has so much other stuff on their plate for WWDC. I suspect there is a clear plan for Quicktime and the future of iTunes and we will find out about both as soon as Apple is ready to tell us.