More than one alias/nickname per user
Kjensen,
First, I'm not sure why the functionality would even be necessary. Why would some people want to find Bob as 123 and some as 456? Wouldn't that confuse people? If Mary and Jane sit next to each other and Gail comes up and says "you guys know what Bob's email address is?" and Mary says "sure type 123 in the To box" and Jane says "no, you type 456"...I would think users would get confused.
However, there usually is a similar need for something like this. While an Exchange account can only have a single Alias it can have multiple email addresses (proxyaddress attribute), with one being the Primary/Sending address.
With Bob here you would create in his Exchange account multiple SMTP addresses:
[email protected] - Set as Primary
This would give you the Auto-Resolve (not to be confused with AutoComplete) functionality you are talking about.