I guess I don't just mean "in parallel" as in "you can install both", but rather a hybrid KDE made up of the old pieces of KDE3 that are working and the new pieces of KDE4 that replace the old. This way, as new pieces become usable and stable they can supplant their older versions.
However, for a variety of reasons KDE major releases (2, 3, 4) are not binary compatible with older releases and thus the KDE team makes no attempt to do this sort of staged release. However for a majorly disruptive change such as KDE4, a staged release is the best way to achieve your goals whithout alienating your users. It is significantly harder, however, from a programming standpoint. If KDE 4.2 doesn't resolve the problems KDE4 has I suspect KDE will lose a lot of market share.