Build 8C154 is labeled iOS 4.2.1 for 2nd Gen Apple TV. Here is an link to the update (267MB DL)
After installation, your Apple TV will show 4.1.1 as the newly installed OS...
Some changes included in the update:
- TV resolution fix for some HD TVs being incorrectly displayed in 480p
- Download fix that removed the annoying "redownloading" of a TV or movie