From scifi.com:

Duchovny Confirms X-Files 2

Former The X-Files star David Duchovny told reporters that creator Chris Carter and series writer Frank Spotnitz are working on a script for a second X-Files movie. "They have an idea, which they like, and they keep threatening to tell me," Duchovny said in a news conference to promote his next film, Connie and Carla. "They're going to tell me soon. They're just setting about writing it now, so we'll be doing it in the next year."

From the pieces he's heard of the idea, Duchovny said that The X-Files 2 would function like a stand-alone episode of the series, with a major guest star. "Apparently, what Chris and Frank have is a great X-File idea with [room for] another actor or actress who can really score in a really great thriller/sci-fi role," he said. "I hope that takes the [film] towards the fans, but also towards new fans."

Duchovny did not address the involvement of Agents Scully (Gillian Anderson) or Doggett (Robert Patrick) in the proposed sequel film. But he assured fans that Mulder would remain unchanged. "Mulder is Mulder," Duchovny said. "That's one of the things I learned fighting for the last three or four years on the show, trying to change the guy or give him a French accent one day. The nature of the character, and what I eventually learned to love about him, is he's set. The great thing about Mulder is we know what he's going to do: what he likes, what he loves and what he hates. We're just going to play with that, I'm sure."