Fudge not setting up-- any salvaging possible? Alternate uses?
It sounds like your fudge simply wasn't heated enough. Fudge is basically a superconcentrated syrup, and it sets when sugar dissolved in the water (from the butter and milk) comes out of solution as the mixture cools and forms crystals. Temperature is your proxy measurement for the concentration of sugar - if you don't hit the right temperature, the concentration will be off and your fudge won't set properly.

Here's a useful little article with some tips for fixing your fudge. Essentially, you want to cook the syrup right up to 237° to 239° F (that's 114° to 115° C) in order to get the proper concentration. I generally heat my syrup up to 235° F and let carry-over do the rest of the work. If it's overcooked (resulting in grainy fudge) or undercooked (resulting in poor setting) all you really need to do is add a bit of cream, reheat the fudge to the target temperature, and let it set again.