With storage devices getting smaller all the time, MP3 players have followed suit--and some have gotten so small that they look almost identical to flash cards themselves.
The credit-card size of a new wallet MP3 player by (who else?) Walletex, which integrates music into the company's similarly sized flash cards, is apparently limited only by the thickness of a USB plug. And Krunker.com notes that it has a "large double-sided branding area for high-quality printing"--a hint at their possible future use as promotional giveaways.
Maybe someday Walletex will do a deal to use Samsung's thinnest LCD so the player can include a screen too. But then again, it probably wouldn't be such a great idea to sit on one of those in your wallet.