With your subscription, you get access to the website where you can download the products and also retrieve keys for them. If you let your subscription expire, you can simply no longer log into the website to download software and retrieve those keys.
So, as long as you wrote down the numbers for the keys already retrieved, you can continue to use them....you simply cannot get any more.
The current promo code that I know of is TNWNF...but this only applies to the Technet Professional subscription which takes the $349 down to $261...it doesn't drop the price of the $199 subscription any.
Edit: From this page,http://technet.microsoft.com/en-us/s.../bb892756.aspx
, you can click a link to "Download the Full List of Products" and this will show you which products come with the Professional subscription ($349 version) versus the standard subscription ($199 version)