For anyone who wants a good primer on how a bunch of bad mortgages led to a global credit crunch, I highly recommend This American Life's The Giant Pool of Money
. It looks like you can either stream it off the site, or buy a download for 95 cents from iTunes.
It's interesting that securitisation played a role in both this, and in the Enron mess. (Securitisation, crudely put, is the process of putting a bunch of assets into a big pool, and then selling shares in that pool.) In theory, securitisation is great, because it lets you have markets in things that are otherwise hard to sell. And, in theory at least, markets are smart.
However, both Enron and the current mess suggest that a big problem with securitisation is that it makes it really easy to bullshit people (including yourself) about the value of what's in that pool of assets. It would be nice if there were a way to fix this, but I'm not sure if there's any regulation in the world that can get around the human power of self-delusion.