I dont think so. It will be confusing everyone. And I'm sure there are developers which use only seam 2 or only seam 3 and dont want to see questions for the other part.
I agree, it will introduce a lot of confusion. Right now, when someone posts about their problem in the Seam 3 forum, we don't have to guess about which version of Seam they're using. Also, while the forums do support tags there is no consistent discipline in using them. Lastly, Seam 3 is pretty much a new framework, mostly written from the ground up. While it duplicates a lot of the features present in Seam 2, it is not compatible (such is the price of standardization).
I'll second Shane on this. Merging the forums would introduce more confusion.
While many of the new developers may be gravitating towards the Seam 3 forums, they are doing so because they want to discuss Seam 3, so then the Seam 2 posters really would get drowned out. There are still plenty of Seam 2 developers, and Marek is around to monitor those forums.
We will get some cross-over once the migration guide gets underway (very soon).