One project I have been meaning to do is digital signature with Acrobat Reader.  Acrobat Reader currently can't sign (breaks its read-only restriction) and verification requires a 'Reader-enabled' digital signature plugins.

While this restriction might make sense for Adobe's bottomline, it doesn't for many web applications.  One could assist/coerce Acrobat Reader to sign and verify, but that requires too mucking arounding with innards of Acrobat Reader as well as PDF.

It would be cleaner and more intuitive to sign the whole PDF using XML-Signature format and route it somehow.  I'll have to find out how companies processing Acrobat forms and figure out a way to integrate.  Adding signature receipt and management feature would be cool too.