Check out Blippr, a new microblogging site with the added focus of being a micro-review site. Review book, movies, music and video games in 160 characters or less! You can find me in the usual place: hieronymus' profile.
They have an api, but it is currently only read only, so you can only fetch data. This means that there are no other clients for it, so you have to post from the web page. But maybe that will change soon.