OmniGraffle for Mac supports this kind of interactive linking (the app calls them "actions"), but we weren't able to fit that feature in before the iPad launch deadline.
If you'd like to see support for them appear in the iPad application, send email to the
support ninjas; they can add a vote to the development database on your behalf. Thanks very much!
iPhone and iPad app design stencils are available at
http://www.graffletopia.com/ and at
http://konigi.com/. Information on getting them loaded into the iPad application is available in
this sticky post.
Hope this helps!