I hope they don't. Last.fm is an app inside Spotify right now, and scrobbling is integrated. All I wish they did is better integration (love tracks, recommendations coming from last.fm instead of whatever they have, etc.), all of which can be done without having to buy last.fm. Last.fm as a standalone service for discovery is good as it is.
Why? How would that help last.fm users?
Last.fm has a great neighbor-based recommendation engine. Combine that with Echo Nest's user-based machine learning approach and Spotify's user base, and you have a pretty robust product
As a grooveshark user who loves last.fm, i don't like this idea.
Why? So they can ruin Last.fm's recommendations with crap music spam in the same way as their own discovery?
Last.fm's integration could be better, but then again most inline apps could be better, so I suspect that is a Spotify issue.