Seagate's extended warranty was designed to apply to drives shipped after June 1, but the company will apparently honor the new warranty for some older drives. The five-year warranty won't apply to external hard drives, though. Seagate's external disks will continue to be covered by a one-year warranty.
Update - Seagate's press release announcing the five-year warranty clarifies a couple of points. First, the five-year warranty applies to all internal hard drives currently shipping in distribution and retail channels. The new warranty also applies retroactively to all drives shipped after June 1 of this year.