The comp.lang.objective-C FAQ listing
Hold Me, Use Me, Free Me (on Reference Counting)
Memory Management with Cocoa/WebObjects
DOM Based XML classes:
NSXML NSXMLDocument NSXMLNode NSXMLElement
Stanford iPhone developer course available free via iTunes U
Open source Mono framework brings C# to iPhone and Wii
Converting J2ME to Objective-C (Commercial, and expensive)
PhoneGap APIs are more generic and can be used on different platforms such as iPhone, Android and Blackberry (but their WIKI also lists Nokia’s Symbian and Maemo as well as Palm’s WebOS).
Titanium is primarily targeting iPhone and Android.
Similar to the iPhone SDK 4, the use of PhoneGap requires an Intel-based computer running Mac OS X Leopard, and you will need to have the iPhone SDK 4 and Xcode already installed.