Always been an interesting theory this one. Mars has been host to some fascinating finds lately further strengthening this belief, here are some arguments and points to consider:
-Mayan predictions for 2012 (new era)
-Hyrogliphics depicting strange alien creatures with humans
-artifacts found on Mars
-human existenece on planet Earth (for 6k years?)
-Water and vegetation on Mars
Here are a few links to explore.