This was my diagram. I started by assigning ip addresses to all links, and loopbacks on the spoke routers. I ran eigrp as 1 over all the connected links, and did not advertise the loopbacks. Then configured the GRE tunnels as follows (I give you one...the other is a mirror):
int tunnel 0
tunnel source serial 0/0
tunnel destination 192.168.23.3
ip address 172.16.13.1 255.255.255.0
After configuring the other side I tested with pings to the other tunnel interface....success.
Then I configured eigrp as 2, and advertised those loopbacks, and the tunnel interfaces (which happened to all be covered by the network statement 172.16.0.0). A show ip eigrp neighbors 2 confirmed the adjancency, and a show ip route eigrp 2 confirmed the updated routes. I can see how this could be useful over a public link, and especially encrypted within an IPSEC tunnel.
I also did the P2P IPSEC SDM config tonight. Not as much fun as the CLI....not at all. But the SPAN setup was cool, and capturing...actually seeing the encrypted data was really neat.