I received an email this morning from Adobe saying that registered users of Photoshop Elements were eligible to purchase Lightroom 4 for $99. However it seems that it is available to everyone and not just PSE users. Offer good until April 27th.
The promo code I received is: LIGHTROOM99
I went to the Adobe site not using the link in the email, clicked on products, then scrolled down to Lightroom 4 and clicked the Buy button. I changed the Upgrade toggle to Full and it showed a price of $99 without ever entering the promo code.