With so many IoC containers is there a decision matrix to help in narrowing them down based on features, supported platforms, etc.?
More is certainly better than less.
There's a good comparison on this blog. And part 2 here.
Another one worth looking at is MEF, it's not strictly an IoC container, but employs many of the same techniques. The important thing to keep in mind is that it is going to ship with .Net 4.0.
I really love the way Nate does stuff with Ninject, it is so clean and a very active project and I would strongly recommend it (even over MEF).
Make sure you look at the history of check-ins for all your candidate containers, if there are few to no updates in the last 6-12 months I would stay clear of them, no matter what the comparison matrix says.
Also, I would look at the test cases for the frameworks, and make sure they make sense. You don't want to be stuck with a over complicated IoC container.