32GB is the posted limit and unless the Evo slot is uSDXC compatible, and I think that's unlikely, there is little chance there will ever be a uSDHC card bigger than 32GB. Sucks but that's the deal!
The newer spec uSDXC should permit cards of 2TB but that will be some years off and it's likely a new card design will supplant the uSD format.
Since I think it's likely that there will be a subset of smart phones that are even larger, physically, than the Evo there is room to use the larger format SD/SDHC/SDXC cards which should offer at least twice the capacity and faster performance due to better controllers. I have the Evo and in two years I hope an even larger phone with 5 inch screen and 1920x1080 resolution is available and that it has a larger card capability be it the full SD/SDHC/SDXC or as yet unannounced new format card of higher capacity and speed.
Brian