After one hundred episodes, the gate will close for Stargate Atlantis. According to star-gate.com news, the series will end in January 2009 after its one hundredth episode, "Enemy at the Gate". A Stargate: Atlantis movie will air at a later date. 

There is a bit of good news for Stargate fans: in the wake of SGA's departure, SciFi has greenlit Stargate: Universe, a brand new series in the franchise.  The series will start with a two hour premiere in 2009.  According to the Hollywood Reporter, the premise of SGU revolves around "a team of explorers who find an ancient unmanned ship called the Destiny. Unable to return to Earth, the crew must fend for themselves aboard the ship, which has a preprogrammed mission taking them to the far reaches of the universe."

SciFi President David Howe said, "This is an opportunity to reinvent this franchise and make it relevant to a new generation.  We really don't want to be more of the same. It's going to build clearly off the existing franchise but with a cast that gives it a younger vibe."

Now taking suggestions for funny new names.  Stargate: Voyager and Three Hour Tour are already spoken for.

(Additional Sources: Gateworld, The Hollywood Reporter)