Since I'm skint I'm afraid I can only afford to round two corners per box. Well, that and I can use the excuse that "Actually, I wanted it to be that way"
I've been playing about. The best article I've found so far is this one: Fluid frame using before: and after: since I would like round corners, but I don't want to compromise my mark-up with loads of extra divs and I'm not clever enough to use javascript to create rounded corners. In theory I could round all corners using generated content (as per that first link), but it has issues with Gecko and since I at least want this to work in Gecko based browsers (I use Camino) and KHTML ones I've opted for just the two corners. It should still be reasonable looking in IE as well (I think one corner will be rounded!)
I'm quite happy with it. It looks alright. It's still a nice fluid layout. I've had to lose the borders on boxes (no biggy) as I haven't figured out how to stop the square border on the round corner and it looks silly like that.
Next I need some drop shadows or perhaps rounded borders. I think this will be really tricky without extra markup though