In the Finder navigate to the /Applications folder and right click on iTunes and select 'Show Package Contents.' Once Contents is open the path is Contents/Resources/English.lproj/TextStyles.plist. Right click on TextStyles.plist and open it with the Property List Editor.
In the plist locate 9003 and click the arrow next to it.
You will see three lines of settings on the 'font' row. I changed the font to LucidaGrange-Bold and on the 'size' line I changed the size from 13 to 18. Note that you need root-level permissions to save the changes.
I now have iTunes with a bold and larger font just the way I like it. I'm a musician and use iTunes to make my band's set lists. I just take a screen capture of the list and print it in Preview.
[crarko adds: I haven't tested this one, but did look at the plist to see that the 9003 is the correct place. As always, editing the .plist file may break code signing and potentially cause problems updating. Make a backup copy of the unmodified version (via Time Machine or other means) before doing this. There may be a way to do this change with a 'defaults write' command, and if someone has that command please put it in the comments.]

