The Dotdeb repositories just (July 11th, 2010) got GPG-signed:
After many requests from several users and after many months of promise, the Dotdeb repositories are GPG-signed. Yes, you can now get rid of the annoying “WARNING: The following packages cannot be authenticated!” message!
Waiting for a dotdeb-keyring package, you just have to get the appropriate file by hand and add it to your trusted keys’ keyring :
The Dotdeb repositories just (July 11th, 2010) got GPG-signed:
Accord to this: http://www.dotdeb.org/2009/03/11/php-529-is-packaged-at-last/#comment-1332 the packages in that repo are not GPG signed so there is no key. Considering that comment is 6 months old now I could be wrong but I can't find anything to the contrary.