I guess the problem here is that if the interface is down the ping will cause it to come up - forcing the ping to be always true.

 If the pppd has a startup delay and you force ping to only send one or two packets, though, then this approach might work.

 My solution would be to have the 'ifup' script touch a file in /tmp, and the 'ifdown' script remove it.

 Then, barring crashes/weirdness testing the connection status is as simple as testing for the existence of the file in /tmp.