This one's apparently had script problems out the wazoo, but Barry Sonnenfeld's MEN IN BLACK III is still on track and happening for a release in 2012. The following pictures were released by The Daily Mail, and definitely go into spoiler territory, so be warned:
According to the article, at some point Will Smith's J goes back in time and works with the young K, played by Tommy Lee Jones in the original film and in this film's modern day scenes, and by Josh Brolin in the 1960s. So there's definitely a time travel story in addition to all the alien hijinks. Brolin looks just like a young Tommy Lee in these pics.
I hope Sonnenfeld got the script situation sorted out; but we'll see in 2012. I loved the first film, thought the second was just there, and hopefully the third film will bring the story back to form.