Shouldn't be a problem, as Kubuntu 9.04 which will be released in 3 or 4 days comes with KDE 4.2.1 or 4.2.2 final. No need for a beta.
(And yeah I agree, 4.2 is the first version in the 4.x series which provides a really smooth user experience. Up to 4.1 it still was a rather bumpy ride.)

Though as far as (K)ubuntu 9.04 is concerned, I'd suggest not to install it on the day of release, but wait a bit. After release of a new version it usually takes around 4 weeks until the Ubuntu guys have fixed all bug found by the early adopters.