Buster Olney tweets that Adam Dunn has been placed on revocable waivers. He also says that Dunn won't clear waivers because numerous NL teams plan to put claims on him, and if any team does claim him, the Nationals can pull him back.
As folks probably know by now, the significance of this is that if he clears waivers he can be traded. Saturday's trade deadline was the "non-waiver" deadline, and any player can still be traded if he passes waivers first at any time this season. Also, any player not on a 40-man roster can be traded, so any minor league prospects the Nationals wanted back for Dunn could still come over.
So basically this means the Nationals are doing their due diligence to be able to trade Dunn, in the event that they get an offer they like. And it doesn't really even imply that the Nationals want to trade him, since players get put out on waivers so often. Jim Bowden used to try to send Ken Griffey through waivers every year, even in his prime.
The one thing about this that's somewhat notable, if it's true (and it might not be, since teams don't have to report waiver moves), is that it means Dunn wasn't sent through waivers earlier in the season. If I understand the rules right (and I might not--if anyone knows for sure, I know you'll tell me in the comments), a player can be traded if he'd passed through waivers at any time in the season. And since they would have presumably had an easier time getting him through waivers earlier in the season, they may have foreclosed some options to make themselves better. (Note--NFA Brian answered my question below, so if you've made it this far, keep reading.)