The reason that most apps' trials use crippling features instead of time-limiting is because unless the developer sets up a server/host and a database to track who is paying and who is trialing, all a person has to do in order to reset the trial is uninstall and reinstall the app. Microsoft does not track trial periods in the store. Therefore, in order to keep people from cheating, we are wiser to use the crippling of premium features that the app could be sampled without rather than have the app disabled after a time period.
WPCentral's app does that. I believe one of the features is toast notifications and live tile that aren't there in the trial if I remember right, but it's been a while....