So the K-On!! Medley in question is the one played by NicoNico artist, Bos. This is a fairly recent video with only 171 views on Youtube and 2,000 on niconico.

http://www.youtube.com/watch?v=UX4lnEMBAKU
http://www.nicovideo.jp/watch/sm10827849

Very innovative medley with a jazz feel to it. If anyone can find some sheet music of this specific medley or can transcribe it, please post it here. I'm almost certain you can't find the original sheet music since it wasn't posted on Bos's video on youtube or nico nico.

I'm still on the look for a transcription of this.

It's all improvised, so no sheets.
Thumbs up for someone to transcribe this song =)

A good part of it is improv and the rest is just skills with a piano/keyboard.^_^ Ill try transcribing it. @_@