Philip Elmer-DeWitt has an interesting article over at his Fortune Apple 2.0 blog that discusses Apple's restrictions on iPad and iPhone giveaways. It's not well-publicized or even strictly enforced by Apple, but the Cupertino company has a two-page policy governing the giveaway or promotion of its iOS devices.
The policy prohibits you from using the word "free" as an modifier of an Apple products name in a prominent way. You still can run an iPad or iPhone giveaway, but you have to use the word free in a less prominent way and submit all the promotional materials to Apple for review. A handful of other restrictions limit how you can use Apple's product images and its font which is not to be used on or in connection with web sites, products, packaging, manuals.