I'm not a lawyer and all those standard disclaimers.
First, talk to your boss. Maybe you can work on iphone apps and release them. You may have to ask for permission for everything you do put out, but it may still be possible. Self-development is a great selling point; you're investing your own free time into becoming a better employee.
Eventually, if you really do want to go indie, just.. quit. Before you do, however, there's a few things you can (and in some cases, should) do.
Save money. After you quit, your savings is what you'll be living on.
Seek partnerships. Other like-minded people may be interested in starting a small studio for you. Even if you're not going to pay salaries for them, knowing people you can trust to do projects (like outsourced art, music, whatever) is a great asset. If you manage to be successful, you may consider hiring them; or maybe they'll hire you. =)
Investigate possibilities for angel investors. Be warned though, taking other people's money always means giving up something, like your freedom. Savings help here.
Possibly, work on something without releasing it. This is where we're far into the I'm-not-a-lawyer category. This depends on how aggressive your employer is and how cutthroat your contract is.
You may also want to check out this seminar by a couple friends of mine. ("Indie Startup for Dummies")