Bob:
There are several other options like the PIC32 based Fubarino Mini(32 Bit, 40 MHz, massive RAM & Flash).
And like you pointed out there is the 5v/3.3v issues. There are also pinout issues, delay loop timing, and more importantly library incompatibilities. Many "standard" Arduino libraries do direct Atmel register access that won't run on other architectures. There are hundreds of posts on the chipKIT Forum from folks having trouble using Arduino libraries on the PIC32 family of chipKIT boards. You can't assume it will work across processor architectures. Some library rewriting/porting may be required. As they say YMMV...
That said, with good Eagle-Fu you could layout your own 32 Bit "Raduino" on an architecture of your choice.
James
KE4MIQ
