What are prime commercial choices for iPhone game development kits/platforms?

I'm looking to get into (as everyone else) the iPhone game development business. I've been programming for a while but have only recently forayed into the Mac development world, although since I've been working with Java for a while learning Objective-C wasn't that big of a deal other than learning memory allocation.

From this, you can safely assume I am not looking for a "teach me basic programming" engine, I want a fully fleshed out engine that can create commercial quality games.

Some of the things I'm interested in:

Good documentation - tutorials and demos - APIs a must
Good support - responses in 3-4 days, not likely to shut down tomorrow, English speaking reps
Extensionable engine - reasonably easy to add new bits to the engine or heavily modify it without having to change 1000 different classes
Working engine - especially since I'm doing iPhone apps which aren't that complicated, I expect most features (animation, networking code, basic collision detection, etc.) to be built in
Reasonable cost - this is somewhat subjective so I'll define: I expect not to have to pay more than $1000 a year for upgrades. The initial cost may be up to $1500 (all required packs included), but if it's more than $1000, I don't expect upgrades to cost more than $500. Upgrades are defined as full version releases, not "content" or "add-ons".
Likely to continue regular updates - I appreciate some companies quality test their code, but I also appreciate companies that release code more than once every 3 years. I expect an upgrade or update (free or pay) every 6 months to a year
Integration with XCode - running the engine with XCode or a full featured IDE is a huge plus. Ease of importing libraries factors in here.

I appreciate all responses, especially those from the voice of experience that would impact iPhone development with a specific engine or experiences with support. If you have a comment about why something is bad (like documentation) please explain what makes it bad and also what a "good" example would be and why that is a better choice.

I will of course be developing on a Mac, so compatability is a must.

Update: It seems the two engines that offer the most are the Unity and Torque engine. Aside from price differences, can someone explain the ease of use of the engines in comparison?

Solution

I did a mini-survey of what was out there back in January and wrote up a post about it. Not much has changed since then so I'll just refer you to that post:

http://ramin.firoozye.com/2009/01/02/building-games-for-the-iphone-and-other-platforms/