What happens when your user attempts to use the app without an Internet connection and it can't phone home to verify it's within the trial period or paid?
Only the first use would need remote access. Assuming that the user just downloaded from the app store, that should be fairly safe assumption.
After the initial connection is made and the trial started, the trial end date can be verified via remote connection but fallback to internal clock for the current timestamp.
Seems like all the parts are already there and it would not be difficult to implement.
You rely on a server for trial functionality, but in the end you have much more control of the system...