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.

This may or may not help the search for answers, but here's a further observation.
My "slow" USB device is running off of a powered hub (USB 2.0) hooked into a MacBook Pro (mid-2009 15" 2.8 GHz).
My other USB device is running off of a MacBook Pro (early 2013, 13"), no hub, and seems to be reporting the correct hash rates to the pool.
I'm going to plug the "slow" one directly into the mid-2009 MacBook Pro and see if it's any different. I'll report back what I find.
Enjoy the time away!
