Well, they were ALWAYS going with a gradual transition to Intel. Most people expect the first Intel-based machines to be PowerBooks/iBooks/Mac Minis.
The professional machines (PowerMacs/XServes) were expected to be last. Possibly once Intel releases dual-core/64-bit chips.
It'll be interesting to see performance specs for the IBM MP chips. Might push some XServe-based supercomputers up in the TOP500 rankings.