Evan, since we're talking typography, a few suggestions...
* Why default the editor to such an ugly font as Courier New? There's no real reason to use a monospaced font at all here, but if you must go monospaced at least put Consolas in front. It would be even better to use a nice proportional font such as Georgia - both in the markdown editor and in the CSS editor within the options dialog.
* Don't limit the font size selector to even numbers when you get above 12. I want Georgia 15! :-)
* The preview window text is awfully small on a high-density monitor like my ThinkPad W510's 145 pixel per inch display. I see how to edit the CSS to make the text bigger (although in a quick test it didn't seem to work at all - will fiddle with it some more) - but with every element sized in px it's a bit of a pain to just make everything bigger. Maybe size everything in percents and just have an overall px size on the body, or some similar technique? Then I could just change one number and make everything bigger.
Would you mind sending me an email, evan [at] markdownpad.com? I'd love to talk typography some more.
Do you edit a lot of markdown text in a font that is not mono-spaced?
I'd like to add that, despite my bitterness, I appreciate having this on windows too, so congrats/thanks for the app :)