We found that Square lets you connect to a Star Micronics receipt printer, and Intuit GoPayment accepts Bluetooth-enabled P25 Blue Bamboo printers -- so those could work for you.

ENGADGET: Ask Engadget: is iPod Touch and Wireless Printer-based credit card processing possible?

We saw a number of Electric Imp demos and appliance ideas at the booth including a simple hand-held "detonator" toggle switch with LED, seven-segment counter, water level sensor, servo-controlled gauge, RGB light, power socket, Christmas light, power monitor, toy washing machine, 16x16 LED display and receipt printer (the latter two being combined to show the avatars and tweets of anyone mentioning the Electric Imp) -- all Internet aware.

ENGADGET: Hands-on with the Electric Imp at Maker Faire (video)