I volunteer with a small charity which locally donates refurbished computers to people who normally could not afford their own computer. Most of these computers now have Linux on them. We are in the process of remastering Xubuntu 9.04 to use as our main distro from now on.

In the past any time a client had a computer problem they telephoned
us and we had to walk them through their problem, or more often than
not, go to their place of residence to help them out, as many of our clients don't even know what panel, task bar, icon, etc. even mean!

It would be fantastic if we could ssh or vpn or whatever into these
machines remotely, at least to check the basic status of the box. However, the vast majority get dynamic IP addresses from their ISP's and some are behind routers also. I'm not really up on networking stuff so any ideas on how this might be accomplished and how to set up our remastered distro with VPN, ssh or whatever, would be greatly appreciated.

cheers...Larry

First idea that comes to mind: DynDNS + SSH + X11 Forwarding
Close second: DynDNS + XVnc server
Third: What's my IP instead of DynDNS + one of the above

that is not a problem
ifconfig will be just fine to find out IP adress

Look into NX. NoMachine NX - Desktop Virtualization and Remote Access Management Software
Simple, fast, easy to configure, uses SSH, so same port 22, and has clients for Mac and Windows.