FWIW, I used WPA and still had that problem before my fresh install.
It seems like nobody can pinpoint the actual cause of this problem other than their upgrade to Leopard, although it doesn't seem to be happening with all upgrades.
Most seem to have issues if they're using a non-Apple router; then again, I haven't had any any problems with my Linksys. Maybe the question should actually be 'is my router crap - was the older operating system willing to tolerate its crappiness'.
I had problems with a Belkin one - solution, took it back, demanded a refund, chucked up a stink, and now I have one that works beautifully with all my machines - PC's and Mac's.