OK, plugged the miner in to a USB2 port before I left for work this morning. BTCGuild is now reporting 233MH/s, so it looks like the miner is working better in a USB2 port.
Must be a USB driver issue...

Yeah, I agree, although it's proving difficult to pinpoint how/why the number of shares produced is lower, especially when the local hashrates are just fine. I've got a few more ideas I'm following up on.
I could be away for up to four days this weekend, but development will resume asap.
