I recently switched from Pivot to WordPress, and took along my favorite theme, Spring, by Andrea Pitschmann, along with me. The new WordPress theme is Springer, and I think it turned out pretty dang well.
It's a 3-column theme that displays, by default, two different categories- default and a linkdump. It uses several plugins (included in the theme) to display recent comments, style the calendar, and such.
You can see the theme in action at my site, Paradoxdruid.com
It has valid CSS and XHTML 1.0 transitional. The transitional is mostly due to the use of
target="_blank", which I know isn't well loved around here. But the design suits me and the other contributors to the blog, and the
target="_blank" is added through a plugin, so it can be relatively easily deactivated.