Probably worried about the ads that gmail inserts that are based on the content of your emails, and that when you delete an email it does not ever really go away with gmail, it's still on the servers... Draw your own conclusions.
I feel that email is not secure anyways, and there are already enough companies out there that probably are probably doing far more invasive data mining than simply scanning my emails for keywords. Course gmail is still beta, and I haven't signed up yet.
Anyways, you can read googles response to concerns here.
I noticed snugtop has though... maybe she could give us some feedback.