I noticed on the MSN website that the new Messenger can chat with yahoo contacts, and then on the Yahoo IM website that Yahoo can now chat with MSN Messenger contacts. I haven't tried it with the official client, but it doesn't look like it requires adding a second account to either program, it looks more like they've connected their chat networks together.
Is this something that Adium can or will be able to support? I tried adding a Messenger contact with the type set to Yahoo in Adium (0.89.1) and it gave me an error.
Not that it's really important to be able to use one through the other as Adium can connect to both natively. I'd like to be able to do it, but I'm resigned to expecting unreasonable things.
Chatting with Yahoo via MSN and MSN via Yahoo
at the moment this works only with the official clients and only with the latest versions, which i think is kinda poor, because i'm sure technically it would have been possible to implement the interoperability on the server side (then it would have worked with all existing clients). anyway, you will have to wait until the protocol changes are adopted by libgaim, which Adium is based on. That might take quite a while.