It's a Menlo world in the new Snow Leopard Xcode. 10.6's Xcode uses the Menlo Regular-11 font for the standard Xcode template. If you miss Monaco (and I know I did), it isn't hard to restore the look and feel of Xcode 10.5's defaults. That's not to say there's anything wrong with Menlo. Menlo is a lovely font. It's just not a familar font and some strange part of my brain keeps freaking out every time I look at the screen.
I'll probably force myself to adapt to Menlo at some point but for the moment, I'd rather just stick with Monaco. So to do that, here are some quick instructions. As you'll see you'll need to create a new theme based on the Xcode default theme and update its font settings.
To start, open Xcode > Xcode Preferences (Command-,). Choose Fonts & Colors. Select the Xcode Default theme and click Duplicate. Enter a name for the new theme (e.g. Normal Xcode Theme) and click OK. Select the new theme, and then select all categories within the theme. To do that, click on any item and then choose Edit > Select All (Command-A).
With the categories all selected, double-click in the font column. A font panel appears. Select Monaco 10 in the font panel and then close the panel. Click OK in the preferences pane and boom. You have returned to a font comfort zone.
Got any Xcode font preferences? Can you recommend a font that's better than Menlo, Monaco, or the good old standby Courier? Let us know in the comments.