Contrary to what people are saying, it doesn't look to be an MS thing. The certificate is not revoked. My guess is that the Phone checks with MS every so often to make sure that the account used to unlock it hasn't expired. Since you unlocked it in the first place by spoofing the server with your PC, it now calls home and gets told that the phone shouldn't be unlocked, and so it locks itself again. Unlocking simply resets the process.
That's my guess anyway. Rafael tweeted to say it wasn't an MS thing either, and that there'd be a blog post coming to explain it...
Contact me on Twitter or through WLM