CMDLET Bidnding Options

I just located this awesome post that details all the various options when doing cmdlet binding. For example, you can define your parameters to a script as they are inputting them. For example:

ValidateScript Validation Attribute
The ValidateScript attribute specifies a script that is used
to validate a parameter or variable value. Windows PowerShell
pipes the value to the script, and generates an error if the
script returns "false" or if the script throws an exception.
When you use the ValidateScript attribute, the value
that is being validated is mapped to the $_ variable. You can
use the $_ variable to refer to the value in the script.
In the following example, the value of the EventDate parameter
must be greater than or equal to the current date. 

Param
(
[parameter()]
[ValidateScript({$_ -ge (get-date)})]
[DateTime]
$EventDate
)
In the following example, the value of the variable $date must be
greater than or equal to the current date and time.
[DateTime][ValidateScript({$_ -ge (get-date)})]$date = (get-date)

I could see using this in a script where I absolutely want the identifier of a user mailbox. Using the ValidateScript I would do a simple Get-Mailbox and validate I get a legitimate answer.