I agree with Sig that it has to do with guild wars, which is WoWs main competitor, but it has more to do with business structure than Blizzard getting worried. The business model for WoW is based on getting more and more people playing to make more revenue through subscriptions, as opposed to guild wars would once you buy, there the shot of cash

. Its reduction is probably timed by their marketing in order to coincide with Factions and the timely release of their expansion. Its done to possibly bring in a few more suckers... I mean customers.
Personally, in terms of pure marketing, GW is a genious new idea, the stand alone continuation. It gets more new people in teh game in waves, while mostly maintaining the old, while pay for expansions tend to dwindle in popularity as the user base gets smaller and smaller with no new infusions lie GWs gets. If I remember correctly (might not be), GWF brought in almost double the new players (multi accounts included of course).
The reduction in price for WoW is by no means a decrease in hope for Blizzard or them getting worried, or even getting slightly unpopular, its merely business tactics in order to maximize their profits.
And in response to SincorpDeath, WoW and GW are about equal in the states, WoW ahead, but worldwide, GW, again last I checked the market, is more or less owning WoW. Its profit is also mostly generated by sales, then subscrip. Subscription mostly pays for server fees and things going back to the players. Which is why I dont like pay for play but thats just cuz im poor.