We were not imagining things, the official Aliens website did have the date as Autumn 2012.
Many other websites reported the news and CVG even managed to get a statement from Gearbox which admitted there was a delay.
Now the official website has moved the release date back to Spring 2012.
Why not just pick a random date in the next couple of years and make your very own Aliens Colonial Marines release date. You’ve probably got as good as guess as anyone when it will be out, including Gearbox and Sega.
Source: Official Site