The new Windows phone 8 app is missing the support for last.fm scrobbling. This can be a dealbreaker for many spotify users and I see no reason why not to support it because it should not be a very complicated feature to add.
We love last.fm and would like to offer scrobbling on Windows Phone in the future. However, due to some prevailing priorities we won't be able to focus on this for some time. We hope to be able to offer this in the future but for now we have to say "not right now". Please continue to add your votes and comments.