UPDATE !!!
The author of the Last.fm wordpress plugin has updated it to make it work !!!

Well, it was…until I found out what was actually going wrong !
« last.fm recent tracks » WordPress plugin by Tijs Teulingsdoesn’t work anymore due to Last.fm new XML syntax.
In the file lastfm.php which comes with the plugin, and which may be located at the /wp-content/plugins directory (relatively to your wordpress installation directory), you have to change, line 92 :
$findme = '<track>'; by
$findme = '<track';
to make it work !
Yeah it’s as simple as that, the XML output of Last.fm has changed and the tag <track> has now an attribute, and often becomes <track streamable= »true »> (which doesn’t match <track> but always matches <track).

If you’re too lazy (or if you don’t want to open a file editor..) you can download the fixed version of lastfm.php

Working version of the lastfm.php file from « last.fm recent tracks » WordPress plugin compatible with the new Last.fm XML syntax