I'm guessing you would replace the native font file with the desired font. Its on the phone and its a normal TrueType font file (I have it installed to Windows and works flawlessly). Its in /system/fonts. I'm not sure which font Android uses as the main system font, since there are others for web browsing and such. There are two main ones, Droid-Sans and Droid Serif. Make sure the new font has the same file name and is a TrueType font. Experiment a little, and report your results back here.
Just make sure you do a Nandroid backup in case you mess up and fonts don't work.