For iphone application development you need to develop skillset in objective C which is built over C and is object oriented. You would also require a iphone device along with a Mac machine running XCode IDE for development.
There are also cross-compilers available like xmlvm but they are in initial stages of development and came into existance becasue of Apple's strict NDA for application development for iphone.
Developing it on windows would be illegal although you can use jailbreaks, windowschain and mac on vm to achive the same.
Also the article is well discussed in forums like stackoverflow to have more insight into the question.
So the minimul investment is the device iphone for testing and deployment, a mac powered PC/laptop for develoment and testing and simulating and last but not the least the intricacies of objective C and going back to good old days of memory management.