I don't think "waiting" is the right term; "working on" might be closer.

With the so-much-more-powerful hardware (which requires bugs ironing out etc.) and new connectivity (mmm, regulation); there's a requirement for a massive improvement in the shipped software to ensure that reviews aren't negative solely on the basis of the software.

New PIM engine; integrated, consistent UI; powerful media players; revamped UI; accelerated graphics; location-aware content; ... - all this takes time to write, and you only get one shot at a lot of press.

Would it have been good for Nokia to release the N900 last month? Yes. Is it practical? No. The next tablet was going to be OMAP3-based to get the performance which has been wanted for so long. The OMAP3430's only just started finalising silicon, software takes time (remember: elephanta was cancelled).