I have been working on a wireless thermal printer for an application that I have in mind.
The system is composed of the following parts:
- Any standard Arduino
- The Texas Instruments CC3000 WiFi shield (https://www.sparkfun.com/products/12071)
- The cheapest thermal printer you can find (https://www.sparkfun.com/products/10438)
The WiFi shield uses the SPI bus, which leaves the serial port free for the printer. In the video below, you can see a simple example of Internet-to-Printer connectivity. As a standalone system with no connection to a PC, the system is started up, and it pings Google. When a successful ping response has been received, it prints the letter “P” with the printer. More information could have been printed here, but since I use an Arduino Diecimila with very little memory, the program only fits on the microcontroller as it is.
The Arduino with the TI CC3000 WiFi shield. 
Thermal Printer + WiFi Shield + Arduino
Here is the code used in the example: