I was playing with various combinations on this page:
http://www.270towin.com/2014-senate-election/flash-map/index.phpAnd I think it really comes down to these three states. If the Democrats hold CO and IA, and if Orman wins in KS (and caucuses with the Democrats), the Democrats keep the Senate.
This simulation does assume that the Democrats hold NC, but it also assumes that they lose the seats of MT, WV, AK, AR, SD and LA. In addition, it assumes that the Democrats fail to pick up KY or GA.
But that's just my opinion, what do you think the Democratic Firewall should be?