Hmm, thanks to this article (via Instapundit), I went digging in the wordpress.com support database and found this helpful explanation. Basically, you just need to enclose the link to the track or playlist in HTML view with leftsquarebracketspotify yourlinkrightsquarebracket. Examples below, taken from the WP support article:
UPDATE: and here is one of my own:
Spin, strangeness, and charm 