I've been using Aptana's Ruby/Rails editors in Eclipse 3.3. It has a 'mark ocurrences' feature that isn't working too well for me. It highlights things well, but the highlights don't go away, and when you save, they often move around, end up highlighting mid-word, including whitespace, and otherwise being incredibly confusing.
Happily, I've learned that they can be turned off here:
Preferences > Ruby > Editor > Mark Occurrences
Maybe this will save some of you the same pain.