The thing you also have to realize is that -- not only do they AoE tank well (and sure, other classes can AoE tank with a bit of effort) they also BENEFIT from ALL shammy totems -- including spell damage, mana spring, windfury, etc.

Not only do they BENEFIT from shammy buffs -- they buff shammies! Kings for PvP, BoW for PvE...

Last but not least, you can start a pally/shammy team at the same level. If you're doing a new team you'll have to wait for your DK to get out of the DK zone (I believe they start at 55 but don't leave the zone until 60?) to meet up with your shammies.

I see the DK as an interesting class, and a playstyle I might like... but I really don't see pallies being REPLACED yet. Let's keep in mind that the pally class is still waiting for its fine tuning from the beta devs, so we haven't really seen much movement or direction in WoTLK regarding that spec.