I have heard mixed information on this.
Personally, if I were you, I would hold off if you can until at least Leopard, see if they update the line with new hardware (LED backlights and Santa Rosa... no whammies, no whammies, STOP!) and I have observed , as others have, that they are killing off all things white in their product lines. They no longer have a white iPod, no more white iMac, and no more white keyboard, so part of me wants to see if they will kill off the white MacBook in favor of something else.
I have heard that in the past, if you were to got out and get a new MacBook, and Leopard releases say 10 days after you get it, you should be able to call them up and plead your case. If you order your MacBook, and Leopard comes out before it ships, then they will delay your shipment to load it on. Also, I have seen this happen, where if they were to upgrade the MacBooks, they will send you the new version.
However, this is very possibly not their Policy.... you know, not the rule, more like a guideline.