It depends on the app.
If your app can be done as a web app then I’d suggest you look at PhoneGap or Titanium or other similar apps. I think you still need a Mac for the final bit but you could easily get the use of one (and if you can’t, ask me).
IF you’re doing games, a lot of your work likely won’t need to be on the Mac – as long as you use an engine which can work on Mac – such as Unity or Cocos or if you stick to C (C++ will work too). Unity for example seems to do everything known to man.
Pop along to one of the XCake meetings in Dublin or Belfast (or more recently, Galway) and meet up with guys who will definitely be interested in helping you out.
http://x-cake.ning.com/