Thank you to everyone who replied -- I feel like a dope, because it turns out the problem was caused by having Groove IP installed, running and logged into the same Gmail account.
As far as I can figure, with Groove IP running on the cell phone, all calls to Gtalk were getting routed there. This included incoming from other callers, as well as calls initiated by me using the Google Voice "Call" button, with Phone to Call With set to Google Talk.
Turning off Groove IP entirely solved the problem. Incoming calls and outgoing calls from Voice now ring the phone. Yea, phone nirvana is back!
I apologize sincerely for not figuring this out earlier, and can only hope others with similar problems see this thread!