One other problem is that even the "barebones" OS will have non-open Nokia blobs in it, unless you don't consider battery charging and GSM functions (among others) important. I personally would.

If you're looking for something that low level, may I suggest you look at the OpenMoko project and it's offshoots. They kind of started out with (and never really got far past) the barebones system. If you want a FreeRunner on the cheep, I may be happy to part with mine if you're really going to use/tinker with it.