Last.fm allows the songs that you play to be recorded. You can download plug-ins for iTunes etc that record the songs you play and then upload the list to Last.fm. You can then embed this list as a widget in your blog so that the whole world (or at least that fraction that reads your blog) can learn about your Barry Manilow problem.
Pandora is a (separate from Last.fm) streaming music service. You might want the songs that you listened to through Pandora to also be recorded into your Plast.fm playlist (in order to de-emphasize the show tunes etc that would otherwise swamp out anything else). Last.fm has an API to allow other applications to add songs but it requires authentication.
The easiest solution would be to give your Last.fm account details (username and password) to Pandora and then have Pandora use the Last.fm API to push your played songs into your playlist there. A variation on this model is to give your credentials to a 3rd party, and have them do the integration between Pandora and Last.fm. Either way, you're handing out your password. This is of course the current default mash-up model.
The other available alternative is to keep your credentials close, and rely on an extended client to authenticate to the Last.fm API and to then push your Pandora songs. Of course, you have to pretty much take it on faith that the extension isn't sharing your password inappropriately (protestations to the contrary notwithstanding)
Everything sent by this extension goes directly to the Last.fm servers and nowhere elseOf course, the best solution would be for Last.fm to accept a security token attesting to my identity in its API and not require the password itself.
You could build some really cool social apps with a 'Playlist Service' defined on top of Liberty Alliance ID-WSF. I'd love to be able to define things like 'If 2 or more friends of mine play a certain song (that I haven't listened to) within the last month, put it on my "Consider listening to" list'. I find that the challenge for me with music these days is how to get exposed to new material that I'm likely to like. I want a system that automates the e-mails I currently receive from friends with subject lines like 'you should give this a try'.
Nobody has yet defined such a service interface for music playlists (AFAIK) but all the social, privacy, and security plumbing is already in place in ID-WSF.