As mentioned, Vince isn't too keen on the match happening for some reason, but he could always end up changing his mind before then. I'm not very interested in the match anymore myself (it's at least ten years or so too late), but a lot of people probably are and with both guys on the north side of 50, a match at Wrestlemania next year is a fitting farewell to both of them given that A. It's being prepared as the biggest Wrestlemania of all time, and B. It's in both mens' home-state of Texas.
There's a rumor floating around that however Lesnar and Taker's match at Summerslam ends, it'll give a strong indication as to who one of them will be facing at Wrestlemania, which has fueled a lot of speculation that Sting will end up costing Taker the match for some reason (that would be a piss-poor way finish to say the least, but you can never put anything past WWE.)