Greetings @Sion2:
I am using the Schlage keypad system with their wireless/Internet module. It can be controlled and monitored by my co-host and me remotely. We can add, delete, or disable codes from any location with Wi-Fi access. A notification is sent to us when the door is locked, unlocked, or jams. We have also set it to automatically lock after 4 min. to "assist" the forgetful guest in securing the rental if s/he neglect to do so.
A unique code is provided for guests and it remains deactivated until AFTER the previous guest has departed and the cleaner has finished her work.
Guests use a variation of their phone number to enter; we can lock/unlock door remotely if there is a problem; and there is a key in case of emergency. No need for the guest to possess a cell phone (hint, hint @Ben). We also use the lock codes to provide entry to our cleaner and repair people.
Let the community know what you decide to use and how well it solves your problem.