ifup-sl does not expect dip to exit unless the connection
fails:

  doexec /usr/bin/dip dip-$DEVICE $DIPSCRIPT
  if [ "$PERSIST" != "yes" -o ! -f /var/run/sl-$DEVICE.dev
]; then
    exit 0
  fi

Unfortunately, the version of dip distributed with RH6.0
(dip-3.3.7o-14) backgrounds immediately after establishing
a connection.

In the case of a successful connection, that means that dip
appears to exit, and ifup-sl assumes the connection has
closed.  If $PERSIST is "yes", ifup-sl will try to
reactivate the connection.  dip starts up, finds the serial
port in use, and exists, so ifup-sl tries to reactivate the
connection...

And so forth.

Workaround: turn off "PERSIST" and think of another way to
handle dropped connections.

Solution: fix dip, or fix ifup-sl.  Perhaps it could
check for the lock file dip puts in /var/lock, and use
this to check whether or not dip is still running.