Hi Steve,
I can not think of a scenario where you can setup a Site-to-site VPN tunnel between two endpoint which have "dynamic IP addresses". If you had one end static and the other end dynamic then you may have a chance.
Having said the above you may want to try the following:
register 2 hostname with dynamic DNS (http://www.dyndns.com/) (I think its free) where you can have a domain name always pointing to your dynamic IP address, then use the hostname instead of the IP address as the peer address.
I have not tried this, but if you have time its worth a shot. let me know how it goes.
Regards,
Michael