According to various sources, K-ON! will be getting character songs. The reasons for this, no doubt, are twofold. The first is because, as a musical series, having more songs for it just makes sense. (See also: Polyphonica). The second reason is because Pony Canyon likes money.
Yui and Mio have their singles releasing on 6/17.
Mugi and Ritsu release theirs on 7/15.
Also, if you see four girls on the road in a mysteriously retro car, there's a bounty on them. Thank you, and good day.
Minor Edit: Switched Mio and Mugi on the release date calendar.