iTunes 10.5 Windows appears to break vorbis playback

Fully updated Vista Home Premium, Dell Inspiron 9100

Until I upgraded to iTunes 10.5, playback worked fine. Upon upgrading, iTunes now seems to ignore any attempt to play back Ogg Vorbis files as if it didn't see the button press. The "Play" triangle never turns to a "Pause" icon, playback never commences. Native MP3 works fine.

I tried removing and reinstalling Xiph to no avail.