Nehalem will not be introduced by Intel until late 2008, and the first processors will be for servers/workstations, so the Mac Pro is likely to see them first. The mobile version (which would fit in the MacBook/MacBook Pro, as well as the iMac and Mac Mini) will not be released until mid-2009.
What is likely coming this summer is Montevina. Montevina is not a processor, but a platform (systemboard). It will use the same Penryn CPUs as the current MacBooks/MacBook Pros use, but will bring some enhancements in memory speed (800MHz instead of 667MHz).
As to your question,
CrossFireXT17, August will likely be a fine time to buy a MacBook Pro. By then Apple will have released new Al cases for the MacBook and MacBook Pro (if the rumor is true) and will likely have made the move to Montevina. I am buying my new one in late July, but that is because I want to take it to the San Diego Comic Con.