OMA Your user account has not been enabled for wireless access. Please contact your system administrator for additional assistance

Running a Windows 2003 and Exchange 2003 if users are having problems with using ActiveSync to the exchange server, I first check that I can login as a user will be outlook mobile access by going to  http:\\mail.companyname.com\oma

If you receive the message upon login

Your user account has not been enabled for wireless access. Please contact your system administrator for additional assistance.

You will see a corresponding event in the event log

Event Type: Error
Event Source: MSExchangeOMA
Event Category: (1000)
Event ID: 1508
Date:  7/07/2009
Time:  10:40:02 AM
User:  N/A
Computer: SERVER
Description:
A user who has not been granted wireless access has tried to access Outlook(R) Mobile Access. User username@domain.local

In exchange system manager go to to Mobile services/properties and check on enable mobile access

Enable outlook Mobile Access

You can see further information for similar issues at: 

http://support.microsoft.com/?kbid=839288

http://support.microsoft.com/kb/817379/EN-US/