4. Create a polished game and approach (or be approached by) an established studio. Also known as the Portal approach. Also the flOw approach. "Sony Computer Entertainment approached some future members of thatgamecompany after seeing Cloud and asked them to form a company and signed them on to make three downloadable games for the PlayStation 3. Cloud ended up being a game that wouldn't be possible for a company as small as thatgamecompany to make, so they made flOw instead. thatgamecompany was created on May 15th, 2006."
3. Work your way up in one or more established studios towards the role of game designer. The American McGee approach. "McGee began his career at id Software. He worked on such games as DOOM, Doom II, Quake, and Quake II in the areas of level design, music production, sound effects development, and program coding. In 1998, he moved to Electronic Arts, where he worked as a consultant on many projects and also created his own game, American McGee's Alice." Mind you, that can be the long route, assuming you're even successful.
2. Work with an independent group of hobbyists and promise to split the profits once you make money. This is difficult to pull off, because contributors lose interest when things become difficult. This is enough of a problem that I'd rather have one paid contractor with modest abilities than a dozen unpaid contributors with spectacular abilities. Blech.
1. Establish your own company and finance development as a third party. Many small developers bootstrap with smaller projects in niche or new markets, eventually working their way up towards larger ones. The iPhone is potentially an awesome way to get your title out there. Start by developing a finished game that's small in scope, and demonstrates the very core concepts of your idea. Rinse. Repeat.
My favorite is, of course, to take #1 and run with it. Tighten your belt, and pay a contractor with good references to help you bring your idea to light on the platform where the competition is still pretty weak, and the barrier to entry is low. That was the Palm Pilot during late '90s, and is probably something like WiiWare or the iPhone now.