Geez, this thread needs some real FACTS to cut through all this B.S....
Blu-Ray = 1080p
Broadcast HD = either 720p or 1080i (network's choice)
"Planet Earth" was broadcast on the HD Theater channel, which is 1080i
HD Theater is available on Cable, Dish, and DirectTVand all of them over-compress their signal, often resulting in annoying digital artifacts.
If you have a 720p HDTV and you watched the series on HD Theater, then your tv was downsampling and de-interlacing the 1080i signal on-the-fly to a 720p signal.
If you have a 1080p HDTV and you watched the series on HD Theater, then your tv was upsampling and de-interlacing the 1080i signal on-the-fly to a 1080p signal.
Because of those factors above, "Planet Earth" on Blu-Ray will have a less-processed, less compressed, less digital-artifact-filled viewing experience than watching it on cable/satelitte...regardless of whether you have a 720p, 1080i, or 1080p HDTV. In other words, it should look spectacular on ANY HDTV with a BluRay player.
Now admittedly, watching these Blu-Ray 1080p discs on a 1080p HDTV is the current "ultimate" setup...but I'd bet most people couldn't tell the difference between 720p and 1080p unless the two sets were sitting side-by-side.