It sure is nice to have an IP address on S0 to ping when troubleshooting. Just my two cents worth. You can use the 10.x.x.x/30 if you need address space for the links.
IP Unnumbered: http://www.cisco.com/warp/public/701/20.html
http://www.cisco.com/univercd/cc/td/doc/product/software/ssr83/rpc_r/48383.htm
Caveats per above link:
Only those serial interfaces using HDLC encapsulation may be unnumbered. It is not possible to use this subcommand with X.25 interfaces.
You cannot use the ping command to determine if the interface is up, since the interface has no address. SNMP (Simple Network Management Protocol) may be used to remotely monitor interface status.
You cannot netboot a runnable image over an unnumbered serial interface.
You cannot support IP security options on an unnumbered interface.
The argument interface-name is the name of another interface in the network server which has an IP address, not another unnumbered interface.
The interface defined by the interface-name argument must be enabled (listed as "up" in the show interfaces command display) for dynamic routing to work properly.
Of course, you have your reasons for going for the unnumbered scheme, my personal preference leans me to go the other way.
