I wouldn't really call it a cliffhanger. The Alien's were destroyed when the ship crashed into Australia and Ripley's story came full circle with her finally returning to Earth. I thought the ending to AR was quite good and had a definite air of closure about it.
That said, i would more than welcome another proper Alien film starring Sigourney Weaver and directed by either Ridley Scott or James Cameron. The people who came up with this reboot idea can go frigg themselves though. They're blatantly only doing it because the Alien and Predator series are 20th Century Fox's cashcows.
I do have a bit of hope for Rodriguez's 'Predators' though, i can't really see him making it as anything less than an R rating. Because let's face it, you can say what you want about films not needing excessive violence, but the Predator films ARE violent and SHOULD be violent, you can't have the universe's ultimate hunting/killing machine as a PG-13 friendly fairy now can you? The creature kills for sport and takes skulls as trophies RAB! To be fair, the same goes for the 'Alien' series too, the whole thing started with a monster bursting bloodily out of John Hurt's chest!