[Update - there's a follow-up video purporting to show limited multiplay here. The Portal gun isn't actually used in this video however, so I'm not sure what to take from it yet.]
Well, not officially, but fans are working out how to make it happen without Valvian blessing. Thinking about exactly how having your opponents suddenly disappear on you and pop into existence on the other side of the map will work makes my head asplode, nevertheless: Woo!
Matt Malesky sends the below short tutorial video he's made of how to get the Portal gun up and running in Half-Life 2 Deathmatch maps. No actual multiplay yet (update - some commentators on this piece hold that this may not ultimately be possible. Ooh, I just don't know who to believe), but this is already waaaay beyond proof of concept, so fingers crossed for the real party soon. Before you watch, a warning: it has got a Radiohead song playing throughout, I'm afraid. I'm not sure why this clearly otherwise excellent fellow felt Oxford's droniest sons were necessary to get the point across, but never mind - just press mute or, as I did, drown it out with your own screaming. Thanks for the tip-off though Matt, and, indeed, thanks for working out how to do this.