Getting Outlook 2007 to use the global address book?

I think you'll find the auto-completion that pops up when typing names in to the To field is not doing an address book lookup - it's scanning a local cache that lists frequently/previously used contacts (from both global and local lists).

Here's a MS support article that explains how to reset the cache (in case of corruption, etc).
I don't know if it will help in your case, so obviously take backups.

You might also me interested in trying Ctrl + K after typing the first part of a name in - it's the shortcut key for Check Names and will either replace the partially-typed name with a full match, if there's only one, or pop-up a box suggesting the various matches in the case where it finds several. However, I've got a feeling (not confirmed) that this only works with the global list, not local.