for the next version release I suggest to create a new view - Last.fm log. It could have simmilar layout as History view (View>History) but it will show songs scrobbled to last.fm. Last.fm original client has this feature also - Its called Diagnose. You can see it here:
As you can see I have nothing cached because I dont use it anymore to scrobble songs (heil to MB
When the song is started, it will show it in the list. When its scrobbled, information is given to this view. When the playback of song ends (it was stopped, next song was played), MB checkes whether it was sent to last.fm and cache it (like its doing it now), when not (MB couldnt connect to last.fm), it stays in this view with attribute e.g. not sent ,until it will be successfully send to last.fm in a load...
I used very often Diagnose view in original last.fm client because sometimes website of last.fm isnt accessable to scrobble so one knew whats going on... Thank God it isnt very often :-)