I've witnessed this too both in WoW and WAR. The recent WoW patch pulled back a lot of players to log in to WoW at prime time at or above peak figures during weekends and holidays, and I've noticed a little dwindling of players on my WAR realm too (and that's a high pop one too).

I'm thinking with the proliferation of games like LOTRO and WAR, players are starting to invest in their hobby a lot more, than say compared to a few years ago, and having more than one MMO sub is not a bad thing. I've seen a lot of people talk about their second MMO, the difference with WoW, is that it doesn't matter if it is the second MMO to the hardcore or the fans of other genres, it is still generating income as the biggest game out there.

I think if the trend was indicative of player habits, the WOTLK will probably decimate WAR's numbers for a short period. Regardless of the quality of WAR, WoW offers something unique to the MMO world right now, a sense of great magnitude, to be part of the biggest online MMO is quite something, and certainly something many want to be part of. One of the biggest issues for Mythic is :

"How many of our subscribers are Blizzard customers?"

I don't see this as an "either/or" argument, both games add something to players, and it's rare for a poster here not to have a particular favorite. I'd like to have seen higher numbers in WAR, as they really do have a mountain to climb to get to the point they can monetize the development in such a way they can bring very high quality content updates to the product.