I'll move to the MBP, or its hopefully, newly redesigned equivilent, in around a year I think. There'll be better chips, cards etc, as there always is. I might even wait until Leopard is released so I don't have to pay for it separately.
(I know, I'm a tightass)
Though I desire the latest and greatest, realistically, my PB should last me at least another 2 or so years, but I know I won't be able to hold out that long.