I saw an ad in a magazine for this, claiming "We guarantee to increase your mpg's by at least 25%".
So, for those who may be interested in something like this, there's a chance you won't be tossing all your money away for nothing.
AquaTune.com - AquaTune Water Injection Systems
My personal beliefs are that H² introduction to intake airflow
may increase combustion, but as of yet I've done no such testing. This setup apparently combines H² and water injection - which could bring different, if not favourable, results.