Yahoo has this deal with SBC (a phone company) where SBC customers are given a "email@example.com" address instead of just using “username”.
If you use the official Yahoo client and add a new contact you will see you will prompted for a messenger ID of the contact and the following examples are given:
It is the only special case that I know of in the Yahoo IM system though. I’'m not certain if this is really an issue, because I do not have a sbcglobal.net address to test with. I am not sure if the address "firstname.lastname@example.org" is the same as “fred”.