I live about 10 miles from Cal Poly SLO and our craigslist is loaded with "commuter" bikes based on old steel MTBs. Problem is people keep asking $200-$300 for them. Apparently the idea has caught on around here.
Ive noticed the trend myself. You reach a breaking point where price for these and prices for something newer begin to crossover. But until that range is reached, these bikes remain a good value for what you get. Up to about $200, as long as the thing is intact - you can hardly go wrong.
I use an old Hard Rock for commuting but I'm saving up for a CX for a little more fun.
Hard Rock here, too. CX bikes are great, if you like the road bike style. Ive come to like the aggressive upright positioning of the large frame ATB's, myself.