Realistically, you need an Intel Mac, the cheapest possible one will do, and access to at least 1 to 3 iOS devices. One device for the current OS release, one device running the OS release of the oldest OS you plan on supporting, and perhaps one dedicated test device for doing beta OS app testing. If you want to do Universal apps, at least 1 iPad and one non-iPad, as there are many things that can't be fully tested on the iOS Simulator.
Going forward, a Mac that can run Lion may end up being required to keep up with the latest SDK frameworks, codesigning tools and app submission tools.
You need to register any iOS devices that you want to use for app testing. They don't need to belong to you or your company. Multiple developers can register the same shared devices.
You don't need to "register" any Macs. You do need to create and/or copy private keys and certificates to each Mac that you want to use for device testing development or app store submission.
Unrealistically, you could get by with none. No devices or owned Macs are required. (There are online tools that will pretty much construct an entire native app Xcode project.) Have a friend or consultant with a Mac get your certificates and do the final codesigning; and let your customers be your beta testers. But the latter could easily lead to apps that get rejected or that crash for most of your customers. And the non-Mac development tools all have various (some severe) deficiencies.