Variable layout (blocks)

[eluser]codex[/eluser]
There's a few sites where you can alter the layout or sequence of layout blocks on a page via AJAX/drag 'n drop. What is a good way to keep track of the new positions? I'm sure it has something to do with reordering the blocks in the database, but maybe this is done differently, or maybe someone has a better idea.

I'm talking about for instance iGoogle.