Lion’s Address Book allows you to select which details to share with others when exporting your contact card. As long as you have told Address Book which card is yours using the Card > Make This My Card option, editing your card will display a checkbox to the right of each detail you’ve entered. Deselect a checkbox, and that detail will be excluded when you drag your card out of Address Book or click the Share button at the bottom.

Previously, I managed two cards: one with all my details that OS X and apps can use to automatically fill forms, including the various email aliases I have for things like shopping and forum threads (addresses that I don’t hand out to friends), and a second for the information I actually want to give to friends and business contacts. Any time something changed (say, a work email address), I had to update both cards. Now I don’t have to anymore.