According to the roster on the team site, Alex Semin has been placed on injured reserve. Of course, that same roster (which, incidentally, is fed to the Caps site by a central register of players) also lists Ben Clymer as a defenseman, so take that for what it's worth, and we'll keep an eye out for confirmation.
Speaking of Clymer, he has been placed on re-entry waivers in an attempt to bring him back to the NHL, likely due to Semin's (and/or Brian Sutherby's) injuries being worse than initially expected, and placing Semin on the IR would seem to support this theory, as opposed to the theory that Clymer's recall is because a trade is in the works. At $1.05m for the next two seasons (recall that a team claiming a player on re-entry waivers is only on the hook for half of that player's salary), I can easily think of a handful of teams who would do well to snag Clymes, but we shall see if anyone actually will. Stay tuned.
Update (10/6): Semin was put on the IR, retroactive to the date of the injury, so he's eligible to come off and play versus Carolina.