What can I do with it?
Add, Remove or Replace items from a property value of an object. This cmdlet can only update a property when it supports the IList interface. So far this does not include any of the core Windows PowerShell cmdlets - however it does include some of the cmdlets that ship with Exchange 2007 and later.
Add additional email addresses to the Test1 user’s mailbox using the Add parameter of Update-List.
How could I have done this in PowerShell 1.0?
Shay Levy has a great blog post on dealing with AD / Mailbox accounts with multi-valued attributes.