K-On! apartment building isn't QUITE a reality, but only barely: Gigazine reports on the themed models of apartment buildings that were spotted at WonFes 2010, which I had not seen any mention of until now.
Along the same lines of itasha (otaku-decorated cars), itachari (bicycles), and other such fan-decorated items, the idea is basically a standard apartment building...but with a big fat piece of K-On! art on the side, as well as on the water tower on the roof. (Well, it wouldn't HAVE to be K-On!, but the particular model being sold at WonFes was.)
In fact, it was really just a clever repackaging of a building model with a bunch of K-On! decals, if you look at it really closely-- but I bet it sold like hotcakes. And it'll soon be available online for you guys who want to start building your own otaku city.
If it sells well, maybe before long you could have an entire K-On! branded replica of Akihabara or something. Would you WANT to own such a thing? And perhaps more interestingly: if you were going to live in an anime-branded apartment building, what anime would it be?