Spotify just announced the embedable Spotify Play button. Let me try to embed what I'm listening to right now: Sara Bareilles - Live at the Fillmore.
Results:
iOS 5.1 + Safari: OK.
Windows 7 x64 + Chrome 19: OK.
Windows 7 x64 + Firefox 11: FAIL "Spotify failed to launch." One of my security extention prevents the content from working properly: NoScript.