To import data from a .pst file, you must run the Import-Mailbox cmdlet from a 32-bit computer that has the following installed;

# 32-bit version of the Exchange management tools (download link : http://www.microsoft.com/downloads/details.aspx?FamilyId=6BE38633-7248-4532-929B-76E9C677E802&displaylang=en )
# Microsoft Office Outlook 2003 Service Pack 2 (SP2) or later versions

a few good articles ;
http://msexchangeteam.com/archive/2007/04/13/437745.aspx
http://www.msexchange.org/articles_tutorials/exchange-server-2007/management-administration/exchange-2007-sp1-mailbox-management-part1.html

Check info:

1.      Please add parameter “-verbose” at the end of the Import-Mailbox and reproduce the issue, which can give us more info to narrow down the cause
2.      Is there any related error or warning event in the application log on the both client and exchange server?
3.    Make sure the mailboxes has been exported on the same version Outlook as you use on your import machine

Troubleshooting:

1. Please make sure that outlook and any MAPI connections are closed prior to running the Import-Mailbox
2. Use outlook 2003 instead of outlook 2007 on the workstation if outlook is to remain open during the process
3. If using outlook 2007, ensure that outlook is logged in using the same credentials as the Import-Mailbox PowerShell window
4. Do not launch PowerShell under the “Run As” method. Instead log into Windows as the desired account with permission to the mailbox and launch PowerShell normally
5. If running Windows XP and getting the error: “PowerShell.EXE has encountered a problem and needs to close.  We are sorry for the inconvenience” – Make sure it dosent get connected to a READ-ONLY domain controller. It will not show up in the log of a XP computer, but trying the same on a Windows 7 and it will show that it connects several times during the import, but if it at one point hits a RODC, Powershell will crash. easy solution just add static entries for the domain to your hosts file.