Here is a good article listing the upcoming improvements in Exchange 2010 SP1 from the Exchange team at Microsoft:

http://msexchangeteam.com/archive/2010/04/05/454533.aspx

Key points include:

Featured enhancements:

• Archiving and discovery improvements
• Outlook Web Access Improvements
• Mobile user and Management improvements
• UI improvements

Plus also there is a rollup of all the existing fixes included