I just had this similar problem happen to me! Although you have probably resolved your issue by now, this is what I ended up doing to fix it.
1. Go to System Preferences and open 'Network' - or click your wifi triangle along the top tool bar and select 'open network preferences'.
2. Click your wifi on the left hand side from the list, and then click the small cog at the bottom of that box, and click 'Make Service Inactive'.
3. Once you've done that, click the minus to delete your wifi as an option from this box.
All other instructions I read said to click the plus and add your wifi again and then restart. Well that did nothing for me, infact it did nothing about 7 or 8 times, soooooo...
5. Restart your computer now, without any wifi configuration setup.
6. Once restarted, go back into system preferences, into network, and use the + button to add a wifi connection back into your computer. With any luck, you will be able to click on 'Turn wifi on' and it will actually work!
I had the same symptoms on my early 2008 MacBook Pro.
The "fix" was a new internal wireless card. That is a job for a service shop.
You might want to follow this post to see if there might be a miracle cure, though: