I'm sure it's like any other Mesa amp that has gone the way of the dodo. If it doesn't have the popularity of the Rectifier or Mark to stay in the line up, it gets replaced by something else.
I know of no special issues with them. The do have to be re-capped, just like any tube amp, but no more so. The DC3s run hot.
Mesa still sells foot switches for DCs.
http://mesa.stores.yahoo.net/dcsers.html