According to this MacAddict forum post, Apple still has a LOT of optimization to do for QuickTime on Intel Macs, despite having more than than half a year to optimize for Intel. The recently released Intel dual-core Macs appear to be slower than even a Mac mini for H.264 encoding:
70 s, Mac Mini G4 1.25
35 s, iMac G5 1.8
94 s, iMac Intel Core Duo 2.0
If these numbers are accurate (and if it's true that QuickTime is already OS X Intel native), one wonders how much of this poor performance is due simply to lack of optimization, and how much is due to the actual capabilities of SSE (vs Altivec).