Almost a year ago I linked to Finale which is an iOS app that listens to the music you're playing and auto-scrobbles to last.fm using song recognition methods similar to Shazam or SoundHound. It works decently well, but it requires two devices: your iOS device (which can't be playing music), and something playing music over speakers. With these limitations its utility is somewhat stunted.
Recently I discovered RadioX 5, which is a Mac OS internet radio streaming application that includes the ability to scrobble plays to last.fm. It has many other features but this one is key for me. It lives in the menubar so it's very unobtrusive. The one disappointment is that it doesn't automatically scrobble songs — you need to manually hit the red circular last.fm icon in the app to save the play. At first this seems like a a huge omission, but without some external way of knowing what is a song and isn't, the application would end up scrobbling non-songs. For example, below is what the application shows when the DJ is speaking between songs on The Colorado Sound (I don't know why the cover art is for Lynyrd Skynyrd, it wasn't played before or after the DJ interlude). I wouldn't want to scrobble the non-song "The Colorado Sound" by the non-artist "The Colorado Sound."
Short of access to a comprehensive database of global artists+songs titles, which if it exists I'll bet wouldn't be cheap for programmatic use, I can think of one way that might allow for auto-scrobbling. If the application had a way to prevent scrobbling for a particular artist+song combination set by the user, probably custom to each radio station, then auto-scrobbling everything else might be practical.