I've tried doing a forum search for this topic, but since the forum doesn't respond well to numbers, I have had no success.

I'm currently running a custom-compiled 2.6.10, and it's working pretty well after a lot of tinkering. What would be the best way for me to keep the configuration settings in 2.6.10 when I compile 2.6.11.5? I would like to copy the .config file, but I believe there might be more configuration options in 2.6.11.5 (and possibly some old options have been made obsolete).

use your old config file with menuconfig or xconfig and then go through the options just to have a quick look, make changes if necessary, save it and compile. theres no significant difference I guess atleast not for your purpose.

I compiled a 2.6.11.5 kernel using the .config taken from the 2.6.10. If you run a 'make oldconfig' it'll only ask you about 5-7 (I don't remember exactly) additional questions where you can safely say 'N'.
Some things though, if you have an ATI (especially Radeon) card, the kernel will load just fine but you might get some errors from the DRM. Also, there's an issue with ALSA which will tell you something about the failure of restoring the volume settings so you'll have to rerun alsamixer & co. And something else, it looks like it's not happening only to me, if you're using exim4 you'll get an "malformed macro definition" error at bootup, problem for which I didn't found a solution so far.
Otherwise, the things went pretty smooth.
Good luck!

Thanks. I ended up using make gconfig and loading the old .config file.