They're in my internets stealing my ID!!!!
I'll wait for you to get your tinfoil hat if you'd like.
Ladies and gentlemen, especially those that are railing against RealID as a privacy/security violation, please settle down for a few minutes and look at what the program is designed to do, and who Blizzard states it is for.
Taken directly from the FAQ, RealID is designed for use with "...people you know and trust in real life -- friends, co-workers, and family -- ....", not DeathKnitexx and Ihealyounow - unless those really are people you know.
RealID is designed to allow you to communicate with folks who would normally be able to reach you via email, phone, etc - without having to use those means. If you trust your guild members enough to hand out your phone number to them, chances are this isn't a real issue.
How it really works
Real ID requires someone to have your personal email (rather the one associated with your WoW account), to start the friend process. Now, since our accounts are tied to our email, you're essentially trusting this person with your login ID - but not your password. And you are using an authenticator, right?
Now, suppose you really want to friend me, and you send a request to manasper at gmail dot com, your incoming request will show up for me the next time I'm online. If I see a request from someone I don't know, I just decline it. It's that simple. You still don't know my real name, you don't know where I live (hint, it's Alaska), and you don't know anything at all really.
Now suppose Whip Davitt sends me a friend request - well then heck yeah, I'm gonna call Whip and make sure it's him - and then go ahead and friend him. Now, Whip has about a million friends because well, Whip will be friends with anyone really, and now all of his friends can see that he knows some guy named Tim Smith. Nobody knows that Adgamorix belongs to Tim, or who any of Tim's toons are - all they can see is that Whip knows a guy named Tim.
So suddenly I get a friend request from Jim Gross, a name I don't recognize. Jim doesn't know me from Adam, but he's a friend of Whip, and wants to be my friend as well. I happen to be a little more selective than Whip though, so I politely (or impolitely, he'll never know) decline his request. The chain ends, and Jim doesn't know any more about Tim than he did before. Sure, he could Google Tim Smith and try and figure out who he really was - but good luck with that one. While you may be able to figure out which of the Tim's is really the Tim you're looking for (and who knows, maybe they get lucky), they still have no way of tracking you in game.
We have ways of finding out who you are
Face it, you're on the internet, nothing is private any more. Now, many of us will use separate emails for everything. I have one for WoW, one for this blog, one for my personal blog, work, guild applications, etc. On top of that I use an authenticator, and I don't answer the phone if your name doesn't show up on my caller ID as someone I know. I'm on Facebook, but I keep my profile limited.
It's certainly possible to find out a lot more about someone than you'd like to think was possible. Ever post on a forum? Chat on vent? I can track your IP address down and narrow your physical location. It doesn't give me your address, but I can start to get closer.
Put your phone number up in the guild info tab? Not only will I know where you live, but I'll know who you are, and I can probably tell you your credit score in a few minutes.
The bottom line
Unless you decide that you want to live technology free, there's a good chance that someone that wants too can find out way more about you than you'd like. At the end of the day, the only thing that will be available via RealID will be your name - and only to people that your friends have become friends with. If you don't trust your friends with your real name, or that they'll never spread your real name, then they may not be your friends after all.
RealID is voluntary. Use it, don't use it, it doesn't really matter. Personally, I won't use it. My friends can all call me or track me down via email/vent/text. Although if we start getting into Starcraft 2 and Cataclsym at the same time, I might reconsider. Even then, if my friends wanted to chat and play WoW/SC2/raid/etc - they'd be online doing just that.
Or they'd call me.
Remember, it's not technology that's evil/corrupt - it's the folks using it.