On the "You had me at EHLO" Exchange Team blog, you can find this excelent Post with PowerShell Examples for E12.
But not all is lost if you still Have Exchange 2003 as much of the tasks listed here, you can also do in Active Directory directly.
I will show here also how you can do this directly in AD using PowerShell, for as you have an Exchange 2003 environment and you need to do this, and so you can see the difference with and without the Exchange 2007 Cmdlet,
I select the mailbox from AD with the PowerShell Active Directory Browser from Last post in the AD series PowerShell : Active Directory Part 10 (AD Browser) , ofcourse there are other ways to get at the mailbox as you could see in that series:
For some more info about setting Active Directory ACL's and ExtendedRights see also PowerShell and Active Directory Part 8 (ACL's) , or do a Search for the GUID : 'ab721a54-1e2f-11d0-9819-00aa0040529b' in Google for more info about SendAs and Exchange 2003 (SP1 + ).