Service that runs once when ppp0 connection shows up

We use wvdial to initiate a cell modem connection. We wrap it with nohup, launched by a cron job that checks if all is well every minute (everything we read said that wvdial and the associated hardware is unreliable enough that you want something like the cron job to watchdog it).

When that network comes alive (always seems to show up as ppp0 from ifconfig), we want to do some work, such as run ntpdate and some rsync operations. Is there anyway to configure a service that runs each time there is a new and established ppp0 connection?