I learned this one back on my first PDA, but it bears repeating. Inside of the address book, you can designate one entry as your business card. When you hold down the address hardware button for a while, the PDA will beam that designated card to a waiting PDA.
The benefit of this is to quickly give your contact information to someone else, without having to give out an easily-lost piece of paper.
If you set up a business card for yourself, think carefully about the information you want on it. Do you really want your home phone/address/cell phone to be given out? Since I am a consultant, my business card lists my work information only, with no cell phone (I never give clients my home or cell phone number – experience has shown them to use it at the most inopportune times!)