what I also would like to add is that it is still be possible to use your dynamic IP to find slingbox.
There is a number of DynamicDNS providers that do that for free with www.dyndns.com being one of them. This method requires a client or a service running on your PC which will constantly update IP assigned by your ISP which can be resolved to let's say yourname.dyndns.org and this is what you use when adding slingbox to slingbox directory. They provide a lot of different domain names so you just pick the one you find easier to remember. This however means your PC must be on at all times.
Another option to avoid having PC at all times is if your router supports it as a lot of them do. This way the router will update dynamic dns provider and you don't need anything running on your PC. For example Verizon FiOS ActionTec, Netgears and I believe D-link support dyndns.com
Some routers don't like bouncing the connections from the private network to the public interface back to the private network. So, It's a good idea to also add a Slingbox Directory entry with the Private IP address and port number. This way you'll have a point to point connection while you're on the same subnet as the Slingbox.