Just noticed minutes ago, I know I am slow... Google has cached my signature information from an unrelated thread, searching for the keyword etc... Is this something new, Ive never know a forum to do that.
Its was my understanding that Google isn't a member of the forums, therefore it doesn't see signatures, when its caching threads. However I see that has changed. Before if you where not logged into the forums, you didn't see users signatures, but now you do. So I guess google see's them too. Maybe this is related to some new feature you installed.
It has always been that way. Signature links have been cached in the past as well and act as backlinks to your website.
Each forum on the net can configure signatures to be on or off by default. DP has them on by default and thus, they are crawled by the search engines. Some other forums have them off by default, but they can be enabled for logged in members. These signatures are not crawled/read by search engines.