Wifi works for everyone but me - I get a "self-assigned IP address" and no Internet connection

Question

I have the most confounding Wifi problem. I just got Verizon Fios, and it works perfectly on my roommate's laptop, his iPad, and on my iPhone. But it doesn't work on my Macbook Pro (running Lion). My Macbook has no problem connecting to other Wifi signals - in fact, there's another connection in my building that I can connect to just fine from my room.
When I try connecting to our own Verizon signal though, the Macbook hunts around for a few seconds, then connects - but says there's no Internet connection. I get an ominous exclamation mark over the Wifi icon. My computer gives me a "self-assigned IP address" and says it can't connect to the Internet.
I've tried everything I can think of: Restarting the computer, turning Wifi off and on, deleting and then re-accessing the signal, turning the firewall on and off, renewing the DHCP lease, restarting the router, switching channels on the router, and more. At this point, I'm out of ideas. Any suggestions?

Answer 1

Please try each of the steps that follow, in order, until resolved:
STEP 1 - Create a New User Account
System Preferences > Accounts
Click "+" to add a new account.
Log off, and then, log back on with the new user account.
Try connecting to a Wi-Fi network with your Mac's AirPort.
If this resolves the problem, there may be something amiss with your current profile and will require further troubleshooting.
If this does not resolve the problem, go ahead and log back into your normal account and go on to the next step.
STEP 2 - Preferred Networks
Delete Preferred Network(s)
System Preferences > Network > Select AirPort > Advanced > AirPort tab
Under Preferred Network, use the minus button to delete the network(s) you regularly use from the list.
Delete AirPort Keychain Entries (Note: This only applies if you connect to AirPort/Time Capsule routers)
Launch the Keychain Access application located in the \Applications\Utilities folder
Click on the "Kind" filter at the top, and look for any "AirPort network password" entries, and then, delete them.
Restart, or log out, then back in.
Add Preferred Network(s)
System Preferences > Network > Select AirPort > Advanced > AirPort tab
Under Preferred Network, click on the "+" button to add any preferred network(s).
Restart, or log out, then back in.
STEP 3 - Move System Configuration Files
Note: You will have to reestablish your network connection settings.
In Finder, go to the Macintosh HD/Library/Preferences folder.
Move (not copy) the SystemConfiguration folder to the OS X desktop.
Restart your Mac. (Note: OS X will automatically rebuild this folder and the associated files contained within.)
If this doesn't resolve the issue, you can move the folder (now on the Desktop) back to its original location.