When Mailbox for iPhone launched last week, it did so with a somewhat unusual "reservation" system. The team behind Mailbox was only letting in a limited number of users at a time, so unless you registered early you were stuck with a countdown of how many people were ahead of you in line to get access to the app. That didn't stop Mailbox from suffering a pretty major outage yesterday, but the makers of new "smart calendar" app Tempo are implementing a similar system. The Tempo team submitted a new version of the app to Apple today, and it'll include a reservation system. Users can sign up for a position in the reservation line, and Tempo will email the users once the app is usable.