iphone-hacks.com

I have an idea for a homebrew application for the iPhone but being new to the iPhone scene I really don't know if there is a way of righting this application. So far from what I have read it looks like Apple is going to open there phone to the commercial market. From what I understand your going to need to register for a developer certificate, not sure what that’s going to mean. Well my question is if there is a way to write homebrews for the iPhone on previous firmware version and is so what would be the best language to write it in (Java, C, etc). Please let me know if this is just wishful thinking or this is actually possible. Thank you