From LedHed's Wiki
Force an update
From the Exchange Management Shell (EMS) run:
Update-GlobalAddressList -identity "Default Global Address List"
References
http://www.computerperformance.co.uk/exchange2007/exchange2007_gal.htm#How_to_Find_the_Exchange_2007_GAL
http://mymcp.blogspot.com/2007/07/how-to-update-global-address-list-in.html