...when it's the latest release from Mos Def.

This blog post from Eskimon has more info.

"In a world where illegal downloads are commonplace, what alternative channels can commercial artists harness?

The music t-shirt has long been a valuable source of social expression; they’re what Hugh MacLeod might term ‘social objects‘.

So making the t-shirt the core product, and providing access to the music via a link on the hang tag, is a masterstroke of contemporary marketing."

Found via PSFK