I have been asked this question many times now - My users who are on (else moved to) Exchange 2007 server cannot see the Free/Busy (Free Busy) information for Exchange 2003 users. ..... read more.....
Cheers
Nitin Gupta (gupnit)
MCITP - EMA/MCTS/MCSE/MCSA/MCP/BS 7799 LA/ITIL v3
http://www.nitingupta.in/blogs