I was thinking the same thing myself. The only problem is that if somebody is say using absolute positioning or some wierd combination of nested floats for their website, will there by any side effects in particular browsers. It would need some thorough testing I think.
Well, my site has a pretty complex layout, and with floating this it is a nested float, and these are my test results using this new method:
Various gecko browsers - perfect.
IE 6 - perfect.
IE 5.5 - For some reason doesnt wanna launch for me anymore.
IE 5 - Works perfectly, so IE 5.5 probably works perfect too.
Opera 8 - perfect
Opera 7 - punbb dissapears completely
Opera 6 - Floats arent handled correctly, punbb placed incorrectly
Various gecko browsers - perfect
Safari 1.2 and 2.0 - perfect
Mac IE - didnt test because my site doesnt work in mac IE.
Opera 7 - same as windows
Opera 8 - perfect.
So, as far as I see only two are bad, Opera 6 and below (which I dont care about, I dont try to support them), and Opera 7 (Id like to support Opera 7, so...).
I'm happy to test any more.
An alternative is to put it in base.css commented out with a note to uncomment it if somebody is having trouble with their website.
I think that should definitely be done at least.