In December 2006 Gearbox and Sega announced that they were working on a brand new Aliens game, having picked up the license earlier in the year.
The game was officially announced in February 2008 and has continually been delayed until a firm release date of spring 2012 was announced.
The official Colonial Marines web site now lists the game as autumn 2012 which, by my count, means Gearbox has been tinkering with the game for nigh on eight years. They could probably have built a fully operational replica of Nostromo in that time.
Source: Official Website