Note: this is a WinterBoard theme. There are tons of tutorials on how to install them
I really love Georgia as my reading font and thus when Reeder for iPhone didn't provide me a choice I decided to fix it myself. A short trial-and-error session with WinterBoard helped to gasp its absolute power and here it is: the Georgia fix anyone can use to read the articles in Reeder with serif fonts.
P.S. In fact you can use any fonts you like with this hack. Just fork it and edit the font info to use the font you like.
P.P.S. I'll try to hack around and transform the whole interface to use serif fonts. Will push an update if I'll find a way to do it safely.