If you cant wait for 32 GB MicroSD cards to show up some time in 2010, you don’t have to. 510Jungleboy have shows us all the way. Sure it involved hacking a gigantic door into your HTC Touch Pro battery cover, but nothing comes without a price (and battery covers are actually pretty cheap).
The hack uses a SD to Micro SD extender card like found here, and allows one to plug in the rather cheap 32 GB full sized SD cards as found here. As can be seen by this screen shot, the OS is perfectly happy with the increased storage.
Interestingly the SDHC standard tops out at 32 GB, so it seems to go any further we will be needing another standard pretty soon.