Most people's computers kick into overdrive when the login screen is open, and you shouldn't have to suffer the extra heat and dragon roars. This modification allows you to slay that e-dragon by changing your login screen, hopefully within the Blizzard ToS (You are not modifying any game files or running third party programs). You can choose from any of the character selection screens, BC, classic, or blank login screens.
1) Put the GLUES folder inside your Interface folder.
2) Structure should be similar to ...World of Warcraft\Interface\GLUES\MODELS\UI_MainMenu_Northrend
3) Open UI_MainMenu_Northrend and select the login screen you want by copying the files from a folder and pasting them into UI_MainMenu_Northrend.
This has been tested with the most of the US and EU clients. You will need to use a workaround if you play on a Mac for some reason.
While people's desire for an 'official' response is understandable, Blizzard is generally loath to comment on things like this. They typically reserve the right to decide at any time that they don't like something, and coming out and saying out that something is okay restricts their ability to do that. Moreover, most of the people who post on the forums are community reps, not the legal team, and strictly avoid making statements that might prove to be legally binding on the company without extensive council.
To address some of the concerns raised as best I'm able:
The game maintains a "folder structure" in its collection of MPQ files. This folder structure is analogous to the folder structure on your hard drive. When the game client needs a particular file, it actually looks on your hard drive FIRST, then if it doesn't find the file, it looks in the MPQ "folders" in the same place. That's how tweaks like this work in the first place. Modifying the MPQ files is very bad; not only will it get you suspended or banned, it will often prevent WoW from loading at all, because a program called Warden checks them. However, Warden doesn't monitor substitute files like gun sound replacements.
The kicker to this is that Blizzard (1) expressly programmed in the ability to override files like this, and (2) later expressly REMOVED the ability to override critical files like the mob and terrain models and textures. This is a pretty good indicator that they're probably okay with it, but it is NOT legally binding and does not constitute actual permission.
Model changing, on the other hand, is well-established as a bad practice and has gotten entire guilds banned. So anything that involves model replacement is pretty suspect.
On the third hand (Quick, get an octopus!), this doesn't impact the actual play experience, which is what they're worried about first and foremost.
Summation: It doesn't look actually bad, but use very much at your own risk; this really straddles a line (yes, I know that's pretty much what the thread has said so far, but I wanted to throw a little more actual information into the mix).