Try to aim for exciting but serious (like kls said, no Ninja talk).<p>My personal bugbear:<p>Make sure you clearly differentiate from things you are not prepared to live without (eg. must have written iPhone app currently on sale at the App Store, or must be prepared to relocate to Outer Mongolia, etc) and stuff that you would like but are willing to forego for an otherwise attractive applicant (eg. having already developed on the iPad)<p>If you aren't specific about the difference between needs and wants, what can happen is that people who do actually have a realistic grasp of their abilities self-censor and don't apply. Then you don't find the ideal candidate and are forced to compromise and hire from a pool of people who think they are object oriented programmers because they took a class in Java once.<p>Highlight anything that would be particularly attractive to a developer (above average compensation, non-moron project manager - rephrase that one - , partial or complete telecommute, longevity of the position, high end computer/laptop provided, flexible hours, etc).
Don't use Rockstar or Ninja! OK in all seriousness, For me at least (before I became a freelancer) talking about the work environment, the people and the app would attract me more than the gory details of what you expect. Most everyone that writes apps, knows what is required of them. I would keep it simple, what you want them to do and what level in their career you want them to be at.<p>As well, I would include what you are willing to pay if it is below market (not that I am saying that you are paying below market) that way expectations are set before time is wasted on either side.