it's not as bad as you think. generally, it works out so that you net a little less threat and a little more avoidance, just due to the way your gear tends to settle out. i have yet to be denied a spot in a raid due to any of my stats being found wanting compared to other tanks. also, the new DW talent in 3.2 is looking like it will bring DW threat in line with 2h, but that remains to be seen.
That's the thing. Just because something "can" be done doesn't mean it is preferable or even reasonable. I won't argue that it can be done, but it's not worth what would be lost in order to switch over to it from two-hand. That's why I'm hoping that perhaps some other things might come into play which would make up for what is lost in the crossover. I'd love to DW tank, but right now I'd be sacrificing a lot of stats to do so.