In the navigation area on the left, next to a few files, on the right there is a little icon badge, "M". What is that for? I read somewhere in the docs that it refers to member symbols, but that's all I've been able to come up with when searching on this subject.

SCM status. M means locally-modified but uncommitted.

Oh, duh... [/facepalm]

I haven't set up source control yet, so I wasn't expecting Xcode to be tracking that.

The default when you create a new project is to use a local git repo. Maybe you clicked through that dialog without reading it?

Ahaaa! Yes, I missed the little GIT checkbox.

Even if you don't tell Xcode about your using svn or git, it figures it out (at least for svn). When I modify my files, it shows the M.

Nice! This is a good feature IMHO.

The new interface and workflow of Xcode4 is taking some getting used to, which are what threw me off with the "M" symbol. I haven't worked with it long enough to form any concrete opinions of it yet, except to say that it's very different. I've been avoiding it for quite some time now, and considering I've already had a crash, my suspicion is that I'm in for more of the same, except now it isn't free.