Gearbox Software Community Forums have word from Randy Pitchford that while the art direction for Borderlands has changed, the conclusion that the game is now cel-shaded is not correct:

Heh! No, Borderlands is not cel-shaded - it's a gritty and serious world after all. But since the game was first unveiled we have made big advancements with the art direction and the technology to support the art and have produced some pretty impressive, even shocking results.

We weren't expecting a story about this to break this week, but I'll tell you that the http://www.borderlandsthegame.com website will soon be getting new content that will expose how far we've gone with the art. If the story you guys broke today makes too much noise for us to deal with we may decide to speed up when we update the site with media. Also, PC Gamer came to visit us recently to check out and play the game with us. The result of that is that the next issue of PC Gamer has Borderlands on the cover and inside there is a big story with a lot of images and a story written by Dan Stapleton. Finally, there's always E3 in a few months where attendees can see the game live. I know some folks have figured out that we've done something big to Borderlands even if no one is quite sure what we've done yet and I know I'm still being a bit cryptic on that, but frankly we aren't quite ready for this story to get out.