Thanks for the replies, laserray, Humax.
Seems that neither leaving out the "http://" nor using stratum, nor any permutation of those solutions works.
I'm really confused because everything worked fine a couple days ago and the same exact command now seems to be failing. I'm still able to join other pools too.
I have seen some "slush" victories on blockchain.info, so I don't think we're down. At least, we haven't been down the whole time I've been suffering this problem.
Could something with my ISP have changed???
Well, funny that you mention it....
The stratum address should work except the pool is experiencing some issues right now (i.e. it went down for a lot of people not too long ago). I'd wait until slush fixes things before giving the stratum address another go. Also you can use direct IP if there's some name resolution issues going on.