Prices are coming down a bit now, but who knows if that will continue or if they will stabilize somewhere soon....or if they'll go right back up. I think that the market for SUV's and other guzzling vehicles has for the most part died and will never again be as popular as it was before prices began to skyrocket. I believe we'll still have SUV's, but only the ones that offer great mileage will survive.
As for diesels I think it's going to take a dramatic fall in prices to get prices back up even a tiny bit. This is the time to buy one if you're wanting one, as I have seen some killer deals on trucks that would be unobtainable by many even as recent as a year ago. I've seen a 72k mile Megacab with a Cummins go for 18,300 bucks and the book was just a bit over 30k! The sad thing is that there still some owners out there who are oblivious to the market and for some reason think they have a truck that is an exception and are asking waaaay more than anyone will pay.