I've come up with a method for generating ASCII character sets for bit-mapped displays using a combination of Excel and a perl script.
The character set is edited "graphically" in Excel allowing you to see what the characters will look like and the perl script converts the Excel data to C source code which can be included in an Arduino sketch.
The characters are nominally 5x7 pixels in size and the C source code implements the character set as a continuous array in program memory with each consecutive character at a 5-byte offset.
The Excel spreadsheet and perl scripts have been tested on both OS X and Windows. The perl script but should work with Linux too but I'm not sure if the spreadsheet will work with OpenOffice.
More information and a link to download the perl script and Excel file can be found here: http://expat.dyndns.org/arduino/character-set-generator/
Enjoy!