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TJ of jQueryHelp.ning.com offers a signature app at very reasonable cost. You can join his site and see how it works.
Hope that helps,
Were you interested in blog signatures for a Ning 2.0 site or a Ning 3.0 site? I sell a Signature app for Ning 2.0 for $100 that works on all replies and comments, but I could modify it so it works on Blog posts only. I am developing a less expensive version that will be out for Ning 2.0 and Ning 3.0 in the near future. Stay tuned!