In Xcode 3.2.1 I have this problem where it stops highlighting Objects in purple, and such. For example:
In that example, NSAutoreleasePool, pool, img, size, w
I noticed that opening projects from the XCode welcome screen causes this issue. If I dismis the welcome screen and then go file open (or file open recent) and open the project that way the syntax highlighting works
Xcode is often glitchy for me in similar ways. Restarting xcode often clears up the issue. Also, sometimes a missing semi-colon or curly brace will keep codeSense and highlighting from working as you want it to. Try Opt-B (build) to see if you have any errors that could be causing this.