I can't really think of a good reason besides updated wifi and SDIO drivers to move to 2.6.. 2.4 has better performance for single-CPU low-memory systems, I believe.
And, let's face it: there's little reason to use SDIO and wifi on a device like the GP2X that has such limited resolution. Web browsing is a pain and if you need console access for development on an F200, you can just get a cradle + serial cable. (Though cradles might be harder to come by and you'd be better off finding a nice used F100 with a busted joystick or something for development.)
It'd be a large undertaking with little reason to do it.