If I was apple, I would announce new macbooks with aluminum enclosures and the shipping date for Leopard around the end of the month, but not shipping them until leopard is out. They don't have much to lose, sales are going to go way down on macs until leopard is out, and at least this way, they keep up a steady pre-order rate.
As for the enclosure, I would make it a three tiered system, much like the nanos were.
Bottom macbook, only in Silver
Middle macbook, silver + whatever colors they would use
Top macbook, only in black
I can always hope this is what they do, and they really do have nothing to lose on an early announcement, and may even gain some pre-orders.
~Thursday