I have been jailbroken on 4.2.1 for over two weeks now. The only time I actually needed to reboot my iPad is while installing critical software from Cydia--such as SBSettings, Backgrounder, etc.
I do reboot every few days just to clear things out... but those are planned and of course I make sure I reboot via RedSn0w. I have never once had my iPad spontaneously reboot or lockup to the point I had to reboot it. (I can't say the same thing about my iPhone.)
My iPhone is a different situation though: that is my phone. So I will not accept a tethered jb on that. And as I mentioned it seems more likely to need a reboot.
But if god forbid my iPad rebooted while away from my PC my iPhone can pretty much do just about anything my iPad can. I try to keep and apps I can loaded on both.
I think some of you are overreacting to the tethered nature. See if you can really go by a week without having to reboot, and I bet you will find you can. OTOH, part of the very reason I can do that is I have jailbreak apps that make it much easier to see what processes are loaded and kill them if my iPad feels sluggish or is not acting right. I don't have this need to reboot just to make things better.
Also remember it's a reboot, not a respring. You can still respring with the 4.2.1 jb... and I probably do that every other day or so, usually when updating something from Cydia. (I would not do any Cydia updates if I knew I was going to be far from my PC--I don't take that chance.)
Michael