No, a netbook is not required, however a supported operating system is. Windows XP SP3 , Windows 7 or Moblin will suffice.
Please download the ATOM SDK, it includes an App Store emulator called ATDS which allows for easy development.
Download here: http://appdeveloper.intel.com/en-us/sdk
You will also want to make use of the Beta Testing feature provided in your Dashboard for more complete testing.
Details here: http://appdeveloper.intel.com/en-us/article/beta-testing
I would also suggest a dedicated virtual test environment. This will ensure a clean test each time with minimal effort to re-create the environment for subsequent tests.
Details here: http://appdeveloper.intel.com/en-us/blog/2009/12/24/how-setup-virtual-test-environment-using-vmware-player-free-solution
Please let us know if you have any questions. Best of luck!
If your application isn't used some Atom platform specific features you don't need to test your application on atom netbook. Just make sure your application window fits netbook screen resolutions.
In addition to the resolution requirements, please review the Validation Guidelines to ensure your application will pass testing: http://appdeveloper.intel.com/en-us/article/validation-guidelines
You are quite welcome. Please post any questions you come up with. Best of luck.