The I guess you missed Unisys, Fujitsu & Bull in addition to HPQ, IBM, SGI & Dell.
Don't forget NEC, they have a pretty decent 32 way I2 box.
Perhaps obsessed Opteron watchers don't realize there is a lucrative market for computers with more than 4 CPUs. Then again when all you have is a Hammer all problems look like nails. ;^)