You learn something new every day!
It appears that ASP.Net's TextBox class doesn't have any built-in validation for trimming the Text property's length if the MaxLength property is set to a value greater than zero.
I suppose this is to give user's more leeway in how they want to validate the control, but it's not immediately obvious that a user can get around this limitation just by altering the html! ie: A user could use the FireFox FireBug extension to remove the textbox input's maxlength attribute and load it up with as much text as they'd like!
So remember kids: Always validate your input!