What would be the most likely form(s) of attack?
This site always shows the 51% attack costs:
http://coinometrics.com/bitcoin/brixbut part of the ASICs they could acquire for free by seizing them.
At this point, this could be done by controlling two largest pools.
Yes, this is a serious risk. If the pool members were blind followers, or individually feel that they have something to gain by sticking in the pool and didn't break off.
Then we all lose. Even them.