They're not "going"... Oddly, I've had the latest firmware upgrade (4.90) on my 660 for a while with no problems. Just did the 2011.20 map update, too, and no problems.
As for "rolling" back the firmware to an earlier version... Yes, you can do that. Too much to explain here, but do a search for the version you want and find the Garmin file. It will self-execute and install. You will lose all your settings as it will be set back to "factory fresh". Be sure to include the term "nuviperry" in your search. That was an archive for virtually every firmware version for every Nuvi.
When you try to start touch screen calibration by holding down the power button (as described in the reply by Garmin posted on 08-21-2010) please note that it will only work if the device is NOT connected to the charger!
The screen will display a notice "Pre-boot mode" and will flicker every few seconds. Ignore this. Just hold the power button down until the calibration screen comes up. On my device it took approx. a minute.
I have both a Nuvi 650 and 660 that both failed within 24 hours of each other. I sent them back for repair, they came fixed with no explanation of the problem. Then 2 weeks later they both failed.
I was able to update them with the latest firmware and they started working again. Then 2 weeks later they failed again.
There is clearly a problem with this series of Nuvi's. I am about to send my units back in again.
Does anyone from Garmin actually read these boards? Perhaps you could help us understand what is going on? It is clearly a software bug as a hardware reset seems to fix it, but I can not figure out how to hardware reset unless I re-install the firmware.