I've done some work today on the “Bloggish” layout, which mimicks TypePad's HTML and can thus have TypePad-shaped stylesheets applied to it. I think Brad's idea with adding this, which I certainly agree with, is to create a nice easy layer atop S2 (S3? ;)) which allows users who know CSS to customize their layout without hacking on S2 code. I imagine an interface where the user can pick what modules he wants in the sidebar, pick a layout and theme and then perhaps also enter some extra CSS to apply. Behind the scenes it'll create a theme and a user layer, with the former containing the stuff that the customize wizard can't deal with such as the list of sidebar modules while the latter just contains the settings which also appear in the customize interface, ensuring that it goes on working as users are accustomed.
For more advanced users who want to do some S2 hacking, the possibility is there to manually write a theme layer and override the sidebar printing functions yourself, giving you more freedom to insert your own special sidebar boxes as you wish. Utility functions are provided to help you generate the right markup to suit the TypeKey stylesheets.
This makes me happy, because LiveJournal has drastically needed this for a long time. That is all. ;)