We saw an infant happily snoozing in one of these in the park the other day so we quietly approached the parent to ask what it was. Turns out it's called a Baby Zabu and is modeled after a traditional Japanese floor cushion called a "zabuton."
The Baby Zabu is the creation of Yuki Mese, founder of Wagamama Baby ("wagamama" means spoiled rotten in Japanese). The Zabu is essentially a padded cushion with sides and can be used for tummy time, playtime, or sleeptime. It also fits into standard portable playpens to become a mattress.
Its portability is it's best feature - it rolls up easily and ties or you can purchase a travel bag ($19) for it. It's made in the U.S.A. and available in two sizes: 36" x 41" ($88) or 41" x 51" ($118) and a nice selection of fabric choices.
Find it at Wagamama Baby.