Well, I saved myself a shit-ton of time last night by being a little patient, unlike my usual self. My PVR150 remote control was completely unresponsive, not just within MythTV, but even using irw to troubleshoot the problem. I was receiving no IR signal at all! I tried restarting all lirc/lircd processes, re-compiling lirc-0.8.2, and fighting with some IR cables, all to no avail. No lirc settings were changed, but I had modified a few display settings (which should have had no effect on my IR device).

Moments away from installing the latest KnoppMyth build, I decided to re-seat the PVR150 card, just for the hell of it. I shut down the system, removed the card from the case and MythTV Setup, and restarted once again. I re-seated the card and re-applied my configuration with success. After reconnecting the IR cable(s) and trying it out, it finally worked! I'm not sure whether the mini-jack connector was grounding out on part of the case (which it could have been, considering how close the card & cable head are to the case edges), or whether the re-seat and re-configuration did the trick, but something fixed the problem. I'd certainly give re-seating a try, even before re-compiling lirc, if you're having hardware/software problems with lirc or your IR device.