I don't think anyone can give that warranty. It all depends on how the app is implemented. There are certain differences between the iPhone type and the iPod Touch and as long as you code the app defensively (i.e. taking care that features that are not available can be overcome) you'll be ok.
The two most obvious ones are:
- not all iPod touches have all features present in the latest incarnation
- the iPhones are always connected
- the iPhone 4 has a second camera - for example, a mirror-like app won't work on anything else than iPhone 4 and latest iPod touch
The best thing you might want to do in your case is to run the app on an iPhone in Airplane Mode.
Also, might be worth looking around for a previous generation version - you can get good deals in the Apple Refurbished Store.