Hi,
natbib defines a set of cite commands that start with an uppercase C (e.g. \Citep), which are meant to be used at the start of sentences, especially if the author name starts with a lowercase letter (as in "von ....").
The current TextMate highlighting doesn't recognise these commands as citation commands. However, this is easily fixed by changing the first occurrence of "cite" to "[Cc]ite" in the language definition.
The fix seems to work fine, so I was wondering whether this should go into the main release.
cheers Hendrik