Cross-platform ways to develop for iPhone

Irrlicht
Engine, SDK, basic editor
License: Free
Runtime platforms: Windows 98, ME, NT 4, 2000, XP, XP64, Vista, CE, Linux, Mac OSX, Solaris, iPhone
http://irrlicht.sourceforge.net/index.html
http://irrlicht.sourceforge.net/phpBB2/viewtopic.php?t=32494

Rhodes
Ruby-based engine, SDK
License: Commercial with GPL option for GPL apps
Runtime platforms: iPhone, Windows Mobile, BlackBerry (RIM), Symbian, Android (Google)
http://www.rhomobile.com/

Game Salad
Beta engine, rich editor
License: Commercial (at least for iPhone)
Runtime platforms: web, iPhone
http://gamesalad.com/

Torque
Highly polished engine, SDK, rich editor
License: Commercial
Runtime platforms: Windows, Mac, Xbox 360, Wii, iPhone, web
http://www.garagegames.com/products/torque-2d/iphone