Actually, it's more than just a USB vs. Firewire issue. It's an issue with video encoding. Sony MicroMV camcorders have Firewire connectivity but do not work with Macs because it uses a proprietary MPEG-2 variant to encode the movies. Their HD camcorders also use MPEG-2 encoding, so those are unsupported as well.
iMovie does not import MPEG-2 from camcorders regardless of connection type. When selecting a camcorder for use with a Mac, it must have a Firewire port
and record in a supported format.
From Apple's iMovie spec page:
Not only does iMovie HD 6 support a vast array of digital camcorders — from standard DV models to solid-state MPEG-4 video cams to state-of-the-art HDV camcorders — but it also supports a wide variety of video formats. That includes DV, DV Widescreen, iSight, MPEG-4 (Simple Profile), and HDV (720p & 1080i).
15" MacBook Pro 2.33 GHz Mac OS X (10.4.8)