@Andrew Nacin -

But why even have Akismet bundled in core.svn, even as an external?
Why not put TwentyTen in themes.svn?
(I won't even get into Hello Dolly... :) )

I'm really trying to understand the thinking. Is there some huge disadvantage to going this route? Does it make it more difficult to package up a default download file (i.e. wordpress.zip)?