Here it is, an extremely detailed and thorough guide to changing the default fonts on the Reader. It's in 4 steps, 17 pages, 9 pictures and 5 appendices. For a procedure that takes a full 15 minutes, it cannot get any more detailed than this. 

If you have any questions, post them here, and I'll get to them when I can.

Dear God, this took me a whole day... and I had so much stuff I needed to do today .

Please carefully read both the Guide and this thread before asking questions! Thank you!

CHANGELOG:

v1.2
- Step Two: parts rewritten to support the next version of PRSC, 1.04; thanks to Jeff for adding explicit support for custom fonts in PRSC
- added licensing info: file under Creative Commons BY-NC-SA.
- some clarifications
- some typo fixes

v1.1
- Step Two: updated references to latest version of PRSC; also changed the screenshot to latest version
- Step Four: it now explicitly instructs people who use Sony's eBook Library to change its fonts too, instead of merely suggesting it
- Step Four: removing and restoring books instruction also made more clear 
- hyperlinks are now blue, instead of that nasty yellow color
- some typo fixes

v1.0
- initial version