I did some GPU testing with high N values. The gist is: at N=8192, I couldn't get it to output valid shares any more. Therefore, with current information, it seems that GPU mining will stop on 13 Aug 2013 - 07:43:28 GMT when N goes to 8192.
a) why do you think it stopped working at 8192 (physical GPU limitation with current gen GPUs, or more a limitation of the code that can be more easily overcome)?
I don't know yet. It just stops working on its own.
b) does CPU mining still work at 8192 and beyond? (if not, then we have a problem on our hand that would at minimum necessitate a hard fork I'd think).
Yes, in fact I tested CPU mining up to N=2^19=524288, where my puny Phenom x6 1055T got around 12 H/s.