I went to one of my notes in Mail.app, selected all the text, brought up the font panel (Command-T) and chose Arial. It gets displayed in Mail.app in Arial, and guess what? When sync'd back to the iPhone, that note is no longer in Marker Felt -- hurray!
[robg adds: You can even edit the note on the iPhone in the newly-chosen font. For those interested, this hint still works in iPhone OS 3.0 to switch the font of a note while still on the iPhone.]

