Seems a lot of people are having issues patching, including myself.
First off, if you're getting "Failed to patch" or some such when you launch WoW, try running the Launcher.exe not WoW.exe (this fixed it for me). Why, I don't know, I've always ran wow.exe directly since before the stupid launcher was created, and the game has always patched correctly every single time up until this one. I think maybe Blizzard tweaked their patch mechanism for some reason.
If the patch downloads but won't patch, you can copy the WoW-2.4.0.8089-to-2.4.1.8125-enUS-patch.exe file, found in C:\Users\Public\Documents\Blizzard Entertainment\World of Warcraft\ on Vista, or on some mirror websites, to your WoW folder and directly run it from there.
One last thing, you might also check the permissions on your WoW folder - I don't see why this would ever change but some users were reporting that their WoW folder was set to read-only and the game couldn't patch.
Hope this is helpful!
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