Good eye - it’s a Mail.app bug, though. You can beta test my UI anytime!

Yeah, I have to live with myself.

The greater spacing between the mail items makes it much easier for me to focus on individual items in that example. I’m not sure how much the difference between icons in the two examples also contributes to this effect.

My eyes see the same as yours. Yes, the Mail bar definitely looks thicker. In fact, I had to read the end of your entry twice to believe that they were really the same size.

I’m considering a status bar that looks like this: http://inessential.com/?comments=1&postid=3345 for my app.

I think it will look more appropriate in Leopard, since they seem to change everything to dark grey.

Anyway, that’s the big gripe of designing interfaces that house graphics beyond your anticipations. You can ‘beautify’ images you will contain in your app with common methods (see Apple’s iWeb; reflections for images or simple Core Image filters on command can help fit things in the general feeling of Macness), and in your situation, an example would be giving the favicon more ‘room’ by forcing it in sub-standard sizes.

There are obvious disadvantages for this, and obvious advantages. As you said, room and negative space plays a huge role in typography, and thus, interfaces in general.

I am quite curious as to where you are going interface-wise with MarsEdit, I’m dying to find out ;).