From Apple’s support site:
Also beginning May 25, security changes will prevent Apple TV (1st generation) from using the iTunes Store. This device is an obsolete Apple product and will not be updated to support these security changes.
The first-generation Apple TV, which came out the same year as the original iPhone, was a weird beast. It ran a version of the Front Row interface that used to be in OS X, and originally was more like an iPod that you connected to your TV: you had to sync content to its internal hard drive. It wasn’t until 2008 that Apple added software to turn it into a standalone device with access to the iTunes Store.
I can’t imagine that there are too many of these still in use—the second-generation and onwards were much cheaper and far more usable.
[Get much more Apple TV-related stuff on our Apple TV page.]