"I completed my 1040 along with all the requisite schedules. Everything was correct and ready for mailing. But... I forgot to include my W-2. That was the only document required. It was mailed a few days before I discovered the error. This has never happened to me. What should I do? Wait until IRS contacts me or will they figure out my W-2 earnings from their data bases?"Congratulations on filing your return so early. You are right; you should wait until the IRS contacts you before sending in a copy of your Form W-2.
In fact, the IRS already has a process for handling situations just like this. The agency will send you a computer-generated notice informing you that you forgot to attach a copy of your W-2. Wait for that notice, and then attach a copy of your W-2 to the response page included in the notice. There will be all sorts of tracking codes on the response page, which will let the IRS agent process your W-2 information quickly.
And don't feel bad about this oversight. Two years ago, I had to amend my return because of a tax document I completely forgot about, until it arrived in the mail. As far as tax mistakes go, I think you'll be just fine.