Try to aim for exciting but serious (like kls said, no Ninja talk).
My personal bugbear:
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)
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.
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).
