Microsoft sent over an official response to the claim by a German blogger that forgot foil was the reason behind the Xbox 360 overheating problems:

The video and photos posted by German blog GameStore24.de show thermal interface pads that are installed per specification. This foil eliminates the need of a protective liner, which simplifies the final assembly process and minimizes shipping concerns and contamination issues. Customers should know that opening the Xbox 360 case voids the warranty. If your console is malfunctioning, call Xbox customer support. They can diagnose whether the system is malfunctioning, and will arrange for repair/replacement if necessary.

There is no systemic issue with Xbox 360. Any customer inquires are being handled on a case-by-case basis. The return rate is below the CE industry average of 3-5%. Of the total number of Xbox 360 systems already in the field, the calls we've received represent a very small fraction. Even so, we're doing everything we can to take care of these customers quickly so they can get back to enjoying their new consoles.