I have a simpler way of reaching the same theory: Islamic terrorists have been responsible for the overwhelming majority of bombings of this nature over the past 20 years, all over the world. I am open to facts proving me wrong - I could also easily imagine this being a domestic terrorist or lone wacko - but when we're talking about using an IED to kill civilians, it seems entirely rational to think first of Islamic terrorists.
I don't think the pressure cooker being used is particularly probative, since I'm sure the James Holmeses of the world are quite capable of researching how to make an effective IED, and the pressure cooker is a proven formula. Also, the use of a pressure cooker has not, from what I've seen, been officially confirmed - it has only been attributed to an unnamed source.