I don't regularly use yahoo messenger, but I'll test this if the opportunity comes. A while back Google (on Gmail specifically) and Facebook both did things where they redirected you to mobile versions of their sites from the N900 - they both stopped soon after, but still. The N900's Yahoo Messenger client may now be being treated the same way by Yahoo, expect in a completely "ignore"/"don't accept connections from it" way instead of mere redirecting. The N900 user-base is certainly perceived as being small enough by the general public for them to conceivably to be dicks like that. I would get as many people as you can to check from N900s. See if you can connect through WiFi and 3G, or if both give the same error. Then see if you can connect to it through the browser directly (surely yahoo messenger has an online version you can use). Report results. If they are consistent across many users, or become consistent over the next week or so, then it's Yahoo's doing. If not, it's something on your end, or in between, or a temporary problem there.