I think there are many shades of atheism from strong conviction there is no God to conviction there is nothing to suggest there is God, just like there are many shades of Religion - from hardcore Fundamentalism to vague Spiritualism.
I'm an atheist who separates the concept of Religion from the concept of God. When it comes to Religion I'm at the extreme end of the scale: very strong conviction that Religion is a complete BS. When it comes to God I'm not at a such extreme end of the scale: I'm convinced that there is nothing to suggest there is God, but I can't say with conviction that there is no God. (for the sake of semantics I'm using the word conviction instead of belief)
So does this mean that I can be open to certain possibilities of God and still remain an Atheist? I will list a couple different scenarios and evaluate their possibilities:
First lets define God: immortal, omniscient and omnipotent Being that created us, can listen to our prayers and perform miracles and remain invisible to us.
Scenario 1: Billions of years ago on another planet another intelligent life evolved (Possible - we know it already happened on Earth). It developed a very advanced medicine that eliminated all natural causes of death (Possible - even we could achieve it one day with advanced antibiotics, antivirals and genetic engineering that stops aging). It spread itself on multiple planets in multiple galaxies (Possible - even with our current technology we could send spaceships that would reach multiple galaxies in billions of years). At that point immortality is achieved - natural causes of death are eliminated and even disasters of galactic proportions wouldn't kill it. Next is omniscience - this is very straightforward - given billions of years it discovers all the laws of the Universe - from the smallest building blacks of matter to the largest intergalactic forces, multiple dimensions or even multiple universes, hence omniscience achieved (Possible - even with us only scratching the surface of the laws of nature we can see that it's possible to reveal all it's secrets given billions of years of research). Next is omnipotence - even more straightforward - with infinity knowledge comes infinite power (Possible - time-travel, travel between universes, terraforming on a galactic scale - nothing is impossible if you have all the knowledge and billions of years to achieve it). Creating us - while working on some galactic projects it decided to plant a primitive form of life on Earth (Possible - the theory of Panspermia already covers it). Listen to prayers - with it's advanced technology and unlimited resources it could monitor any activity on Earth (Possible - even knowing our thoughts is possible if you have advanced technology to record the electrical impulses in the brain and piece them together). Perform miracles - anything we couldn't explain with our limited knowledge of science would seem a miracle to us (Possible. On a side-note - the bible lists as miracles such mundane tasks like curing the blind as miracles - our medicine is already doing it - does it mean we have achieved partial Godness?). Remain invisible - that shouldn't be a problem with such advanced technology, I can come up with many ways of doing it: hide in different dimension, manipulate the electrical impulses in our optical nerves so that even if we look straight at it we wouldn't see ti, etc. (Possible - have you played hide and seek with an one year old? If you are much smarter you can make sure you are never found). Final result: God.
Scenario 2: Our world is not real, we exist in a virtual reality program (like The Matrix) running on a supercomputer somewhere (Possible - we already have many computer games with similar idea, like The Sims). For all intents and purposes the developer of that program is a God to us - immortal, omniscient, omnipotent, created us, can listen to our prayers, perform miracles and remain invisible.
Scenario 3: Similar to Scenario 1, but instead of a life form evolving, it is an artificial intelligence that developed. It is immortal to begin with, because it's not alive, it becomes self-aware and follows similar path as Scenario 1 (Possible - even Richard Hawkins has warned that artificial intelligence could spell end of human race).
I came up with these 3 God Scenarios in just half an hour - I'm sure many more could be written by a good science fiction writer. Are they possible? Yes, they are, just as any other science fiction is possible, but to start believing in them is pretty farfetched. Which reminds me - there is already a science fiction religion - Scientology was founded by L. Ron Hubbard - a quite popular science fiction writer at the time. Maybe in the future we will have a Star Wars religion, Star Trek religion, Dune religion, Ender's Game religion, etc. - any of them would be more believable that the current, badly written, archaic fiction religions.