Ceramic frit pigments can now be applied to glass surfaces using inkjet technology which opens up printing to the digital imaging world. There are several companies out there selling flatbed inkjet ceramic frit printers. The one I find most information about is Dip Tech and their GlassJet series of printers - http://www.dip-tech.com/sitefiles/1/294/1613.asp

Here is a video about the GlassJet - http://www.youtube.com/watch?v=gaihUL5aKaM

Cheers,
David