There is an issue with Exchange 2010 Personal Archives support for Outlook 2007 which was included in the December 2010 Cumulative Update for Office 2007. This could result in users being unable to access their archive mailbox.
A fix for this problem is expected to be included in the February 2011 Cumulative Update for Office 2007, which will be released later this month.
For more information about the personal archive feature, see Understanding Personal Archives