that's funny... The Sun Fire is barely twice as fast as the xServe, with 6 times as many processors! It probably costs a pretty penny too. The amount of xServe's that you could buy for the price of a Sun Fire 4800 would probably kick the 4800's @$$ (well, maybe, it depends on how clustering works.)
It's good to see that Apple is kicking some dell butt in the server market, since that's where dell makes most of its money. If Apple introduced a whole line of servers, from small, affordable units to bigger, faster, more powerful servers, they could probably be a threat to dell...
Only time will tell what Apple's going to do...