The Redskins decision to trade out of the 21st pick is clearly the right choice. Trading the pick for Chad Johnson would clearly have been a disastrous mistake, and they're lucky the Bengals didn't let them make it. Instead, by trading with the Falcons (along with their 84th and 154th picks), they get: the 34th, 48th, and 103rd picks. With no wide receiver taken thus far, the Redskins are in great position to take one in the 2nd round. Phillip Merling may still be available as well. Instead of having to decide between the two, the skins could get Merling & Malcolm Kelly, two guys they were looking at very closely.
Trading down, in general is a good idea. The draft is a regular crapshoot. For every Peyton Manning, there's a Ryan Leaf. There's also plenty of star players that have been picked in later rounds. By trading down, you get more picks, so you can increase your probability of finding a star.
The Redskins have a team that made the playoffs last year, and have quality players at most positions (not a lot of "stars", however). They don't have an absolute need, aside from a tall receiver and some D-line help. By trading down and getting more picks, they can draft for depth, so they can get some quality help when players go down later in the season.
In a few years, we may look back at this offseason for the 'skins as one of their best, and a turning point in the expensive-free-agent-oriented front office. They now have 9 picks, pending any additional trades