Besides danah's questions, I think the most painful point here will be the duplication of effort and the inevitable integration. Two development teams, two codebases, two networks, two admin teams, and one checkbook can spell trouble unless you have billions. 6A is smarter than the average cookie so I expect they'll do fine and I also realize that they did not make this decision lightly (if anybody ever does).
Also, if you're interested in building communities (or accidentally find yourself with one), you should take some time and read what danah has to say.
# — 06 January, 2005