Question: Some years ago, I picked up a Cannondale R3000 and moved all the components to my current bike as an exercise in bike building, never repeated (in fairness, the Cannondale fit me a little better but my beautiful 1998 Klein has such a sweet, sweet paint job...). I've been putting the frame on Craigslist off and on for a few years, but no one seems to want to pay what I'm asking. Regardless, my current MTB is an all-mountain beast that I bought before I knew my riding style, and I'm thinking of picking up a more cross-country steed.

Times being tight and all, is there anything preventing me from building out the Cannondale road frame as a mountain bike? Obvious [potential] answers (e.g., tire clearance) are good, more involved ones (e.g., frame robustness) are too. I'm trying to get a picture of what might lie ahead in this venture.

Answer: 
Converting to a straight-up mountain bike would be hard. Converting a road bike to a 650B bike capable of handling dirt trails and gravel is a pretty established practice, mostly entailing swapping the wheelset and brakes:
<http://www.cyclofiend.com/cc/2010/cc769-joehuddleston0410.html>
It's more usually done on older frames, but nothing necessarily precludes doing it on a newish racy frame. However, a narrow chainstay/seatstay/fork gap may make fitting fat tires difficult.

However, you'd be able to pick up a decent used mountain bike for the price of a decent 650b wheelset, especially if you'd be fine with a rigid bike. $125 or so can get you Tange Prestige with early 90s Deore LX and the like.

If you want a project for your R300 frame rather than selling it cheap, there's always a singlespeed conversion (the Wabi wheelset is great for the price).
I would strongly advise against converting an aluminum Cannondale road frame to mountain bike duty. They're not built to take the headtube stress a typical suspension fork will inflict on the frame. If you keep a rigid front fork and go the cyclocross route, you'll have better luck.