The new WotLK client has a few new graphics settings. Try setting shadows to absolute minimum. Anything other than that for me and I get a single digit FPS instead of my usual 40-80.

Blizzard made a bit of a mess of the game's graphics since patch 3.0.3 (3.0.2 ran fine). For example there are threads in forums with 20+ pages and no solutions, about the game pausing for a fraction of a second every 15 seconds, and players who have run debugging software on WoW have found bugs in the program that Blizzard should have been able to find themselves (ones that cause page faults, causing the game to load data from HDD instead of memory, causing the pauses every 15 seconds).

PTR looks to have fixed this bug, so it might get fixed in 3.0.8.