I've bought the Serial version of the device, and there is no Gnu/Linux drivers for this version, only for the USB version and the Ethernet version. I finally made my choice a few weeks ago and bought a Star Micronics TSP600 which is perfect !. A new TSP100 series printer (thermal receipt printer USB) can be found for $200 / USD or 150€.
Some of these are thermal receipt printers in about the right price range. If a legal expert or Star Micronics can clarify the license then it is probably safe to assume that all or at least some of the following printers can be used safely: 'The GNU General Public License or GPL, under which this software set is freely licensed to you. This is the CUPS printer driver package, containing:' The readme seems to suggest that the complete source is under the GPL although they have also included a proprietary license in the folder.
The problem with Star Micronics is that the licensing is unclear.
I spent quite a bit of time looking into this issue and it would appear Star Micronics has made a serious effort to support GNU/Linux and free software (HP doesn't have a solution). I'm pretty confident Epson is not the way to go.
