If you deploy Microsoft Outlook 2010 during your Exchange 2003 to Exchange 2010 migration, please be aware that Microsoft Outlook 2010 does not use the direct booking code that is used in older versions of Outlook and you have resources enabled to use direct booking.
To fix the problem, we manually modified the registry to enable direct booking as we only have several users that use a few resources that are enabled for direct booking.
For more information, please refer to Microsoft KB Article 982774.