from http://bugs.musicbrainz.org/ticket/5620 and assuming still valid

Drag & Drop from iTunes (9.0.3) in "Snow-Leopard" not working. But fles from finder can be added normally.

falcotron replied with

I played around with this a bit. The problem is that what iTunes lets you drag isn't a file.

It's theoretically possible to create a handler for iTunes' special clipboard/drag format that pulls the location property out, turns it into a pathname, and opens it. I think Finder can do this (or maybe iTunes does the hard work specially for Finder?), but almost no other app can, including just about everything from Apple. Try dragging MP3s from Finder into TextEdit, QuickTime, or GarageBand; now try dragging from iTunes.

There's probably a good reason nothing supports this, and I don't think Picard should try to do something QT, GB, etc. won't.

A Picard feature to open the current iTunes track/selection/playlist would be pretty easy, and might be useful. An iTunes script to do the same thing is even easier, once http://bugs.musicbrainz.org/ticket/5786 is fixed (see the link on that bug page). Neither of these is the same as drag-and-drop, but I think they'd get people what they want.

and again with

First, as far as the workarounds go: * I added a plugin to do the "Add from iTunes" stuff to the plugins list. * The #5786 fix is checked in, so the next version of Picard (0.12.2) should work with the iTunes script I mentioned. * I'm working on an extensive patch (see http://picardplugins.googlecode.com if you want a very early alpha that may crash your computer, run up your Visa bill, steal your girlfriend, and/or eat your grandmother) that adds more complete iTunes integration.

But, as I said, none of this is the same as drag&drop.

The good news is that I did a bit more experimenting, and I've discovered that receiving (useful) drags from iTunes isn't actually impossible. The bad news is that it's not easy, and may be next to impossible from a Qt app like Picard. If you want the gory details, see the "Pasteboard" section at http://code.google.com/p/picardplugins/wiki/AddFromiTunes (and be warned, they are gory).