For HD video the GH1 is superior to the D90 in almost every regard. However, as good as the GH1 is, it has some unique gotchas, and also limitations shared by all video-capable DSLRs, as described in the posts above and elsewhere. YMMV.
Hi Austin: Links to sources for relatively inexpensive lens mount adapter rings for the GH1 are in the GH1 FAQ on DVXuser.com and in my l-o-n-g thread on Cinematography.com, which I linked to in my post above.
DVXuser.com has several DSLR video-related forums.
There are also lots of folks here on the COW who have hard-won info to share about these cams, too. Maybe the COW will add a forum specifically for DSLR-for-video someday, but in the meantime there's the more general Digital Photography forum here: http://forums.creativecow.net/digitalphotography
Correction: Unfortunately, the B&H link in my previous post no longer points to an extremely detailed review written by DP Glenn Przyborski. I don't know if Glenn deleted it or if B&H took it down, but it's unfortunate it's gone.