The thing with Astonishing is, it's so late, and so out of context with the other books, that having it participate in any crossovers would be useless, as it's chapter probably wouldn't come out until 2010. Whedon and Cassaday are pretty much being left to their own devices until they're done (and I'm fairly sure the follow-up team of Ellis and Bianchi are being treated the same way).
As for Messiah Complex, I'm interested in it, to the extent that I will Byrne-steal the books in the store, but I probably won't be buying any. I liked Carey's X-men run (or at least the first story arc. It was culled in the "I have no money!" cutback), and Brubaker is apparently just hitting his stride on Uncanny now. I love Bachalo's art too, but Ramos I can take or leave. I'm more intrigued by the rumors of what the fallout from the event will be.