Does it only work with WLM? I tried with the old MSN Messenger and it does not work...

It's only with The new Windows Live Messenger or Yahoo! Messenger with Voice.

I just signed up for the interoperability feature, and since then I can't sign into WLM

same here I Just cannot login

UPDATE:

I can log in through the WebMessenger client, but not with WLM
uninstall/reinstall does nothing. I think the change in protocol may have hit a snag, and WLM could be suffering connectivity problems as a result (but not WebMessenger, if it's not using the new protocol)

EDIT:
Apparently, although I can log in with WebMessenger, I'm not visible to my contacts, no matter what my status is. I can still talk to them, though.

Yea I made a post about this and I can login through Trillian but not WLM

well i had the same problem but i just found out it was my firewall blocking it (zone alarm security suite), so i just got a new security suite (pc chillin) and it fixed it

Edit: and i switched suites because non of zone alarms solutions were working for me

It works fine for me.. although its not that good lol, the only thing you can do it chat/ignore and nudge

I have ZoneAlarm, and it STILL doesn't work when I disable it.

I know it's a local problem, I can get it to work on my 2nd computer without a hitch. I'm going mad here

I created a new account for the hell of it, and I can login fine. So I have a problem with one specific account on this one specific computer (my main account on my main machine - just my luck!)

ive picked up a Bug in it.. after you add someone, you have to Logout an Re-log back in to get the Notificaion , but bare in Mind, its still a  BETA process