As in TextEdit and other apps, you can use Option-Escape to complete certain fields in the Address Book. The possible completions for a particular field will be based on the values of that field for your existing contacts.
This isn't particularly obvious, because the 'Complete' option doesn't appear on the Edit menu like it does in other apps, but I was tired of retyping the same cities and company names, so I figured it was worth a shot.
It's not perfect, so I suggest you experiment to see where it works and where it doesn't. It would be nice, for example, if it could be used to complete names for the spouse or manager type fields, but since a person's name is actually multiple fields from the Address Book's point of view, it isn't able to complete those fields in a useful way.
[robg adds: This is a great timesaver, and something I'd never thought to try...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060315064320703