I've seen several Death Knights already at 80 on my realm (and to my everlasting horror, two of them in my guild beat me to 80), but haven't noticed any real difference in the number of tells in trade and LFG searching for a tank. Whenever I see these, it's hard not to wonder what's going on. While it's much too early for the majority of leveling DK's to have had a big impact on 5-man tanking between 70 and 80, most of the ones I've seen at 80 are rerolled DPS and have continued in that role. Some would like to tank but just don't have the gear to withstand the damage of a high-level dungeon. Others have privately admitted that, while they're willing to give tanking a try, having to learn it at 80 with an impatient group that just wants to get through a dungeon is a daunting prospect. Still others really do just prefer to DPS.