Generally, any time you develop a product that is made to "work with" another--especially a wildly popular one--you need to work out some form of license arrangement.

In the license arrangement, you pay for the privilege of having your product associated with the other's (in your example, the iPod). The reason is that you are banking on your product selling well precisely because you’re riding on the coattails of the other product's success. In other words, it's the association with the other product--such as the iPod--that’s contributing to the cache of your own product. Otherwise, why would anyone care about your product?

Consumers want the Belkin carrying case because, first and foremost, it carries their iPod. Also be aware that large companies such as Apple also have large legal teams and large budgets for ferreting out companies that use their trademarks without proper compensation. It’s not enough to have a "disclaimer" that it’s not an official Apple product.