I'll give you my non-SEO advice here as all the answers above are good from an SEO perspective. I personally don't believe in them. From a security standpoint they don't actually prove anything. I can show you 1,000's of insecure sites that carry big name trust badges. Whenever I see a site prominently displaying a trust badge I always think they are trying to compensate for a lack of security with a giant sticker.
On companies you would generally trust like microsoft, vmware, google, amazon, etc there are no trust badges displayed. I think a nice statement in your policies about security is a much better move. People who are already security conscious can seek out the information while users who weren't worried about it in the first place won't be freaked out and security snobs like myself won't think you're trying hard 
Ps. feel free to drop me a line if you need any security advice.
-Nick
