Microsoft shot down speculation floated here, issuing an official statement:
The reference to a 100 GB hard drive in a recent presentation in Korea has been misleading and in fact we have no plans to bring a 100 GB hard drive to the market. We apologize for any confusion this may have caused.
I’m not sure why you’d need 100 gigabytes in an Xbox anyway, unless you wanted it to be a media server or something else. That doesn’t seem worth the effort when you can buy Media Center PCs with bigger hard drives and a DVD burner for under $400.