This is one of the basic necessities of being an admin and using any type of scripting. How do you read content from a file? For example, you have a text file with usernames that you want to migrate to exchange 2007. Well powershell makes it easy for you.

$a = Get-content c:\usernames.txt

This will read the file and save the contents into an array called $a. Now if you want to use that to move a mailbox, try the following:

Get-content c:\usernames.txt | move-mailbox -targetdatabase databasename

This commands reads the usernames from the files and then pipes each username one at a time to the move-mailbox command. A quick and easy way to move a group of users from a text file.