Personally, I would wait for the next update. Althought I usually would agree with the "buy it when you need it" mentality, there seems to be much that needs changing in the Macbook to warrant waiting.
For one, Apple seems to be phasing out their "white for consumer" idea, and there is no other product in Apple's computer line that is white anymore, meaning they may make the new MB an aluminum one as well (perhaps with a black option?). Also, besides the 17" MBP, they are the only line with no LED backlight, something that seems long overdue since the last update (SR, but no LED). The lack of it in the 17" may be a cost issue, but the avaiability of one in the MBA suggests that LEDs shoudl be in the MB soon. Lastly with the introduction of the multi-touch trackpad, it seems to be something Appe will be bringing soonder than later to all their notebook computers.
Again I agree that technology is always changing and that you shoudl buy something as you need it, I believe that the next update should be a big enoug one to consider waiting a tad bit longer.