Lately, my d pad has often stopped responding. This is inconvenient in general, and particularly when I'm in "lock touch screen and keys"?

That happened once to me. In fact, everythingstopped responding except the screen and power button. When I rebooted it stopped operating and I had to reflash. Sorry I don't haveanything hepful to say, but you are not alone.

This is not a hardware issue, in your opinion(s)? If not, is it sluggish in other ways? If so, it could be something hogging the CPU or eating all the memory or something.

It seemed like a software issue. Otherwise, rebooting would not likely affect it.

Newbie n800 owner of 2 days and this has hapened twice.
Had autolock enabled, key/screen locked, when I came back could not unlock as center of d pad would not work. My cure/workaround was to plug into ac to charge. d pad then responded. I should have had enough charge in the battery, but it's possible this is a factor. For now I have disabled autolocking and will see how it goes.

I've had this happen twice. I don't remember how I got it out of this state the first time. The second time I found something small enough to lever out the battery and that fixed it.

It feels to me like some kind of software deadlock.