validate form element based on other element value

validate form element based on other element value

I have a dropdown with an option for 'Other'
 
I would like it to validate the text box (and display an error msg with the rest of the validation) for being empty IF the user selects Other from the dropdown. Also it would be nice if the text box for the Other was hidden until the user selects Other from the dropdown.
 
I am using the usual jquery rules and messages, but i have no idea how to do this.
 
Cheers
Darryl