Harold Wong is a Senior Technology Specialist at Microsoft and a great Technet presenter. He has done recently some great webcasts about Exchange and also some Labcasts. I recently had the opportunity to attend one of his Labcasts, Exchange Server 2007: Compliance and Retention Labcast (Level 200).
Harold was recently interviewed by Let's Talk Computers in February about Exchange Server 2007. If you are interested in listening to the interview, please go here: http://www.lets-talk-computers.com/guests/microsoft/exchange_2007/index.htm.
By the way, I have Harold's Blog included in my blogroll. Read it, it's great!