As far as I know, all iterations of Core 2 Duo are 64-bit. While Tiger only supports 64-bits for UNIX apps, Leopard will be 64-bits all around.
I'm just interested if anyone has received their new Core 2 Duo MacBook Pros yet? I ordered mine around October 30th, and it's now November 8th and there still isn't a single unit to be seen in any stores around here. I was told that a total of ONE unit was being shipped over to Australia today, and another 13 tomorrow...I was hoping to have it by the weekend, seeing as it's my birthday and all.