Whilst creating a child theme based on the default WordPress 3 Twenty Ten theme, I found it useful to create a Photoshop version of the themes’ layout as a starting point. You can download it for your own use here.
Screenshots
Appearance matches that of theme as rendered in browser:

Where possible, layer names reflect corresponding HTML elements and #id or .class attributes:

Home page and single post/page included, can be toggled using Layer Comps palette:

Complete range of typographic styles presented as editable text layers. Styling for threaded comments also included:


Credits
Sample content adapted from that provided by WP Candy
More info on the Twenty Ten theme can be found Here
Licence
Creative Commons Attribution 3.0 Unported Licence
In a nutshell, you are free to copy, distribute and alter this work for the creation of WordPress sites based upon the Twenty Ten theme, but please don’t just republish the source PSD or derivatives of it without linking back to here in some way.
If, by copying the layout of the Twenty Ten theme, I am upsetting people back in WordPress HQ, please let me know, as that is not my intention.
Thanks!

October 26, 2010 at 2:32 am
Thank you for sharing, this photoshop file very useful.
February 11, 2011 at 2:02 am
Thanks very much for this, exactly what i was just sitting down to build myself,
thanks for saving me tons of work!
February 11, 2011 at 12:32 pm
No problems Mike! Glad to be of help.
February 24, 2011 at 1:55 pm
Thanks for sharing the PSD.
April 12, 2011 at 11:19 am
Thanks a lot! Very usefull PSD.
April 13, 2011 at 4:51 am
I wanted to do it by myself, but then I google and found you. Thanks!
May 16, 2011 at 1:46 pm
Thank you so much for this ressource ;)
September 4, 2011 at 12:09 pm
Thank you, this will help me getting started with my ideas^^