Little bit of time on Virium tonight, highlighted a couple of buglets in Slick which I've since fixed and committed. I've added aliens and generators (that pump out the aliens), heres a screenie:

Can't shoot them yet, thats next - maybe with particles. We'll have to see.
Oh, and check out GameContainer.setMinimumLogicUpdateInterval(). It wins the prize for longest method name ever but is extremely useful. It allows you to limit how often your game logic (update() method) is called. This is very useful once it starts to get heavy - say collidng a couple of hundred aliens. I'm currently rendering at 1100 fps but only updating the logic at 100 fps. Animations run independently of logic so they stay as smooth as the frame rate (as does a bunch of other stuff :))
UPDATE: Ok, here's a link to
Virium demo webstart built, packed, webstarted and uploaded with the standard Slick build.xml.