SongTweeter is an app created by Japanese MVP Iseebi, and is designed to tweet your currently playing song to your twitter followers.
What makes this app unique is that it supports Japanese text entry, using Social IME, a web service that can convert Hiragana to Kanji. To enter Hiragana mode, you have to type the dot char (right of minus, left of backslash).
Of course you do not have to be Japanese to use the app, and despite the screen shots the app is fully localized in English.
It comes with a free trial, with the only limitation being not being able to edit the default format (the tweet can itself still be edited).
About the only feature missing, which I hope the developer considers implementing, is a shortcut to the Zune Marketplace version of the song, so that Zune subscribers can instantly sample the tweeted track. It also does not integrate with the Zune hub yet. It appears already to have somewhat of a following, as can be seen by a hashtag search.
The app is $0.99 and can be found in Marketplace here.