Andrew,

I have a Microsoft Exchange server account through my web host, 1and1.com
The Exchange server allows me to access Calendar/Address book info on any
computer, and as you state - with the same information, no matter where I
entered it from. However, it is a single account - meaning all of the
connection info to the Exchange Server is the same and it only comes with one
email address. However, that aside there are so many features to Exchange
that I have yet to play with and if your company is able to secure it's own
Exchange server you'll have options galore to mess with.

Hope that spurs you on towards a solution.

Joseph