Quite annoying but:
I’ve just noticed that my MacBook drops its wireless connection when on battery power and connected to a Netgear DG834 router.
It only happens when on battery power and the internet connection is idle for a few seconds.
SOLUTION: Either do not run on battery power OR if you must do so do not let the internet connection run idle.
IN PRACTICAL TERMS:
1. Keep your Macbook plugged in.
Now, using Leopard, I notice that it disconnects periodically and then reconnects which is better than before but, not great. Not sure if this is an Apple or Netgear problem.