Deep breath, it's okay. What's happened is that somehow you have ended up with a beta version of iTunes on your computer and it needs to be replaced by the production version. Most likely, you're an iPhone developer and were playing with early release firmware for the phone: when I was testing iOS4 for my iPhone, I had to upgrade to a beta version of iTunes for full compatibility. A few weeks later, the app started to complain that it was going to expire.
I kinda freaked out too, honestly, until I remembered that it was a beta copy.
Here's what I saw with my beta copy of iTunes when it started to complain:
The solution? Download a new copy of iTunes! That's it.
Start here: iTunes download link.
Make sure your old copy of iTunes isn't running and the new version will install over it. Go through the installation steps and the first time you launch iTunes you'll see a different message, one requiring you to agree to the iTunes license terms. Click on "Agree" and... problem solved!
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I've seen messages like that before with beta/pre-release/release-candidate versions. However, the message usually says it's beta/whatever, and includes a link to get the latest version. I can definitely see how that particular message would freak people out.