I wish Adobe would just go ahead and make this a permanent deal, but for now the Photoshop Photography Program, which includes Photoshop CC and Lightroom 5, for $9.99/mo has been extended through May 31, 2014.
There are no limitations for prior software ownership to join at this rate (as there have been in the past). You can sign up for the Photoshop Photography Program here on Adobe’s website.
(At the time of this post, Adobe’s site still has the offer end date as March 31; however, I have official confirmation from Adobe that the deal is extended through May 31.)