The Script:

$ sudo cat /usr/bin/linkevdo
#!/bin/sh
if [ -e /dev/evdo0 ]
then
        LINK="evdo1"
else
        LINK="evdo0"
fi

echo $LINK

=================
The Rule:
KERNEL=="ttyUSB*", ATTRS{product}=="Sierra Wireless AirCard 580", PROGRAM="/usr/bin/linkevdo", SYMLINK+="%c"