This is probably one way to do it (sorry if this has been found out already)
If you were to use say this format http://www.minecraft.net/play.jsp?user=&id=0
Then it would load the first saved iteration of the person's map in singleplayer. This of course replacing with someones username (case sensitive)
and the id bit at the end telling you what saved iteration you are going to.
so if I wanted to play on Snowman's first saved iteration in single player all i gotta do is type this into my browser
again I have no idea if this has been said or discovered before and my apologies if it has
but none the less its an interesting way to look at peoples singleplayer saves
Before MP was released, this is how people made maps together. One person would start, then save, and give the link. The next person would go on, build some things, save, and pass on their own link. Repeat until needed. Griefer free too, since if someone did any damage, it was only their own maps they saved.
This has actually been known since the very first day we had level saving. Notch told everyone. And everyone freaked out because WE HAD LEVEL SAVING! WOO YAY WOO.
But I guess you folks are too young to remember those days. Back when water didn't exist. Or clouds. Or trees.
I remember that, but now that there's multiplayer it pretty much died, a while ago somebody tried making a community level, but that died too. It's sad really, that way it was IMPOSSIBLE to grief. oh well.