This is a helpful application that doesn’t exist yet, that I know of.
It works inside Mail. It lets you select an email message, or text inside an email message, select something from a contextual menu, and have your Address Book automatically updated with the phone and address in that email or text.
Even better, possibly, is an application that can scan all your emails and find the ones with text that looks like contact information (numbers formatted like phone numbers, and “Street” or “Drive” or variants). It maps the sender addresses to your Address Book entries, and offers to change them all en masse.
Even better than that would be an application that, in addition, does this when you first receive email, and makes the changes semi-automatically. By “semi-” I mean it presents you with exactly what information it’s going to change first, and allows you to edit what changes it makes or cancel them individually. (Something I’d really like to see more of in Sync Services, by the way.)
Time and time again, I look up someone’s contact information in Address Book, and it’s outdated. They’ve sent me an email with the new information, so I have to hunt for it, and do the tedious cut-and-paste from one application to another.
I would pay for this application.
Better make it fast, though; sounds like Audigone material.
Weaponized Fluff 
Outlook, at least, lets you right-click on e-mail addresses to add them to your contacts.
For everything else, it’s just a matter of forcing everyone to use vCards 🙂
<tooting-own-horn>
Don’t forget Anthracite! It’s not exactly what you’re asking for (yet), but I’ve been using it internally for a very similar application, scraping the Kagi sales receipts in the inbox for the customer info. This example starts with an AppleScript to grab the current e-mail text, then runs that text through a grip of processors to get the individual fields we want, and then finally stuffs those fields’ values into both a MySQL database for safekeeping and HTML table for review.
http://www.metafy.com/products/screenshots_kagi.html
You could certainly use one of those regexen to look for phone numbers, for example, and then of course, parsing the variety of different signature blocks is all part of the fun!
To get the data into Address Book instead of MySQL, you’d replace the MySQL Export Object in Anthracite with an AppleScript Export Object and you’d be most of the way there.
Thanks for the links, see you at WWDC again next year!
Joe @ Metafy
</tooting-own-horn>
Sounds like you want SBook-style contact information intelligence, right inside of Mail.app. It would enable stuff like providing a contextual popup when clicking on an email or postal address and saying “update Address Book entry”.
http://www.sbook5.com
Wasn’t there some technology that Apple was working on back in the day that was supposed to do something like that? It’s right on the tip of my tongue…
Buzz is thinking of Internet Data Detectors. I miss those. 😦