"So, Tyrion and Bronn bromance series?"
Development deals, out of favour in Hollywood during the financial crisis, have recently come back into vogue. J.J. Abrams has such a deal in place, allowing him to develop or produce new shows for several networks (two pilots based on his ideas have been greenlit for later this year). Abrams's deal allows him to develop TV projects whilst working elsewhere: he has been working full-time on his second Star Trek movie for the past eighteen months or so, and is about to plough into working full-time on Star Wars: Episode VII for anything up to three years.
On that basis, fears that this deal will delay The Winds of Winter (the sixth and currently-planned-to-be-penultimate Song of Ice and Fire novel) until the middle of next decade seem somewhat overstated, though it is possible will have an impact of some sort. It's also possible that this deal was made as a sweetner to keep Martin on-board with GoT, or that HBO are actively considering adapting some of Martin's back-catalogue and this allows them easier access to it.
In particular, there has been speculation that HBO are planning to adapt the Dunk and Egg prequel novellas as an ongoing (and perhaps more episodic) TV series. These stories - The Hedge Knight (1998), The Sworn Sword (2003), The Mystery Knight (2010) and The She-Wolves (forthcoming) - are set between 85 and 90 years before the events of ASoIaF/GoT, so would not impact on that ongoing main series. HBO have also done some subtle foreshadowing for the series, with Old Nan mentioning Ser Duncan the Tall to Bran Stark in a Season 1 episode of the show. It was recently confirmed that GRRM's prior deal with HBO did not include Dunk and Egg, so a new deal such as this one would be required to adapt them. With at least one major new character in the later novels set-up much better by the prequel novellas, it is possible that HBO are thinking about going down a similar route.
There has also been speculation that HBO might consider adapting the Wild Cards TV series as their own take on superheroes. However, SyFy and Universal own the Wild Cards TV and movie rights, so HBO would either have to co-produce or buy the rights outright from them, which would be unusual in Hollywood. A TV or film adaptation of Fevre Dream or one of Martin's Thousand Worlds SF stories and novels also cannot be ruled out.
Martin has yet to comment publicly on the deal, but will likely soon do so via his blog.