This looks to be a long dead thread, but I figured this one out when the Pre Minus first came out, I'm a bit of a font enthusiast. It requires a bit of dirty work, a bit of terminal, but here you go.
1. Take whatever .ttf font set you want to use and copy onto your internal space.
2. Rename all of them according to the naming scheme used by webOS. WebOS stores the fonts in /usr/share/fonts/ and are easily found using Internalz Pro. They are named Prelude-Medium.ttf, Prelude-Bold.ttf, etc.
3. Go into terminal (I can't figure out how to copy them using Internalz Pro. Can it change permissions...?). Type 'cp -f Prelude-Medium.ttf /usr/share/fonts/' Do this with all the font files.
4. Restart the device. Tah-Dah! Your new font is being used!
I had to fall back on my Pre, as my Epic died and I'm waiting for it to be fixed. In the meantime I decided to update my fonts from the Ubuntu Family fonts I had installed, to Google's new Roboto font. It looks great!