If you want to write a native objective-c cocoa touch app, you need a mac.

However Novell and Adobe have alternative solutions:

here is info on mono/iphone dev: http://arstechnica.com/open-source/news/2009/01/open-source-...

here is info on flash/iphone dev: http://labs.adobe.com/technologies/flashcs5/appsfor_iphone/

You could create a hackintosh, or install mac-osx in a VM. None of these are officially endorsed solutions and I have no idea how stable these hacks are.