A problem that I have been running into a lot while working with Xcode 4, Git and team mates is trailing whitespace. For some reason Xcode purely sucks at adding whitespace, causing team mates to see edits during code reviews. My team uses Reitveld for code review, and making that experience any less sucktastic is worth some effort.
A quick way that I found to show spaces and tabs in Xcode 4, before you commit, etc., is to use the Show Invisibles feature.
This will toggle your code view from normal, to showing tabs and spaces visually.
I also mapped this feature to Command+9 so I could quickly switch between showing and hiding invisibles.