This triblade concept of linking an Opteron with Cell over PCIe sounds highly interesting. But I doubt it is an SSI, is it?
I'd really wish there was SSI hardware with different CPU types. That could aid in software development, or just for the fun of possessing one. Something unlike Cell though (SPU/SPE are all in the PPC group, kinda boring) what I think of is more like a mainboard with, say, one AMD64 core, one IA64 core and a SPARC64 core.