I just upgraded from the "stable" to the "testing" distribution (in order to get a version of FireFox that is compatible with GreaseMonkey, but that's a different story), and now the X server will not start. During boot I now notice an error saying that udev is not started because it requires at leaset a 2.6.15 kernel, but I am running a 2.6.7 kernel. And in the X log file I see errors about not being able to open devices. Anyone know how to fix this?

Welcome to linuxquestions

You answered the question already. Install new kernel which version is 2.6.15 or higher.

You could also tell your distribution you're using...
I'm quessing Debian though...

Thanks for the help. I'll try upgrading the kernel. Must I recompile my own kernel, or is there a debian package that I can just install via aptitude (or apt-get)? (Yes, I am using debian; sorry I forgot to mention that.) How do I know what version I should use? I see several packages with the word "kernel" listed when I search:
[oops, I am not allowed to post the URL until I have posted 3 messages]
Can I just install one of those using aptitude? If so, which? This one,
[sorry, had to delete the URL again] ,
for example, says that "It does _not_ provide a usable kernel for your full Debian system."

There are many things that can go wrong with an "upgrade" from Sarge to Etch, mostly related to the conversion from XFree86 to Xorg. I maintain that it's better to do a clean Etch install. In Testing the kernel packages are named "linux-image-2.6...". I think the current one is 2.6.17 or 2.6.18. Get both the linux-image, and linux-headers files with apt-get or aptitude. After that, reboot and do an apt-get dist-upgrade just to be sure. If your X server still doesn't work, ... ask again.

I followed your guidance and that helped, thanks! The kernel upgrade seemed to go smoothly and the udev error is now gone, as are the errors about being unable to open devices. I used "aptitude install kernel-image-2.6-686" and "aptitude install linux-kernel-headers", though apparently I already had the headers. It took me quite a bit of web surfing to figure out what architecture my computer has (Compaq nc6000 with Pentium M processor), but I eventually found info on wikipedia indicating that a Pentium M is a 686 architecture. (Shouldn't there be an easier way to determine one's architecture?)

HOWEVER, I then got a bunch of "Could not init font path element" errors, culminating with a Fatal Server error from X: "could not open default font 'fixed'". I searched the debian packages for fonts, took a guess at installing the "xfonts-75dpi" and "xfonts-base" packages, and now it works! Hallelujah!!!

Thanks very much for your help!