Slashdot thinks that even putting the sticker on a device is against the law.
FTC Warns Manufacturers That 'Warranty Void If Removed' Stickers Break the Law
The original FTC warning letter doesn't seem to support the conclusion that using "warranty void if broken' seals is illegal - but breaking the seal does not void the warranty.
FTC Staff Warns Companies that It Is Illegal to Condition Warranty Coverage on the Use of Specified Parts or Services