I recall having some problems with Xcode's syntax highlighting and autocompletion in the 3.2.x versions. I don't remember how I resolved that issue, but it did go away.
I've been using Xcode 4 GM for a couple weeks and yesterday started experiencing a major issue with the code sense autocompletion.
After cleaning and rebuilding my project, syntax highlighting seemed to work. As soon as I started to edit a file, though, the highlighting was dropped and any attempt and autocompletion returned no suggestions.
Looking around, I found suggestions to delete the DerivedData folder to force a rebuild of the Code Sense index (you can do this from the Organizer, on the Projects tab in Xcode 4). You can also turn the Quick Help off.
I aslo tried restarting Xcode, cleaning, rebuilding, etc.
The problem continued to plague me. I ended up creating a new project and copying my files from the original project. It was a bit of a pain but so far it seems to have fixed the issue.