i dunno if you're much of an npr person or if you listened to the may 9th episode of this american life that, i thought, explained the mortgage crisis really well:

"the giant pool of money":
http://www.thisamericanlife.org/Radio_Episode.aspx?sched=1242

but if you did, and found it to be a clear and concise summary of how it all came about, they had a followup episode last week, this time explaining how the mortgage troubles spread, what the actual problems are (beyond the usual and unhelpful "banks and investment firms are failing"), and what that $700 billion bailout is supposed to do:

"another frightening show about the economy":
http://www.thisamericanlife.org/Radio_Episode.aspx?episode=365

totally worth checking out if you give a hoot about this sort of thing. . .

oh, and fyi, if you haven't listened to the mortgage one first (i.e., "the giant pool of money"), you should. . . it's not crucial to the understanding of the second episode (i.e., "another frightening show about the economy"), but it does provide a useful backstory. . . y'know, kind of like how having watched "happy days" made one appreciate the subtle genius of "joanie loves chachi" all the more. ..