The crop factor does make it very interesting. Like you say, a 70-200mm f2.8 becomes 134-384mm f2.8 (equiv) Pretty damn incredible for sports and wildlife. Throw on the much loved tc1.4II and you have a 187x537mm f4 (equiv). Wow... Just for an extra wow, the 300mm f4 AFS with the TC1.4II becomes an 806mm F5.6 (equiv)! Yes, you have to suffer with only 15.4 MP. Tripod sales are going to increase.
Of course, we'll be reaching optical limits of the TCs.