21 Mh/s for a CPU is not bad.
Its only got 6 working SPE's. How many stream processors does an ATI video card have? hundreds!
If IBM ever released the 4 PPU + 32 SPE Cell, given the current cellminer (which I think could be optimised further if it was written using stream processing) it would do 115 Mh/s.
Is that mind shattering? No its not, but I think its still remarkable given that everyone likes to compare the Cell to a GPU. TDP would be around 100W, but alas we are talking about a product which will not see the light of day unless Sony decides to stay with the cell for PS4.