Fastest way from Photoshop to theme?

You are in a common predicament.
What I normally do is pick an existing theme that has the same general layout (number of columns etc) and then spice it up slowly by removing and adding things. CSS is difficult at first but once you get to understand it, it's more powerful than HTML layout. I work with designers who create artist impressions of the site, and I translate it into working CMS/Drupal websites.

Use themes on PHPtemplate, they are easier to modify and understand.
Ofcourse if you need it all done externally and you are willing to pay for my weekends, I can get it done .
Otherwise I can guide you as much as possible pro-bono in the spirit of Opensource

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Web Development, Production & Marketing Advice - http://www.cmsproducer.com/click/26/3

I use Adobe Imageready to export my photoshop designs. You can export the layout as an html file. There is also an option to use CSS divs instead of tables.

After you have the html file, you can modify it by referencing from an existing template.

You can build table based drupal themes but I find it harder and less flexible than CSS. It took me many hours to learn to build table less layouts that worked in all the major browsers.

What I do now is I have built 2 plain html css layouts. One fixed width one fluid, and once the design has been finalized I pick the layout that is the closest and build a static html css page. Once I got that 100% working in all mayor browsers I open the phptemplete Bluemarine or Box grey themes and copy past the relevant php into my theme and I am done.

If it is urgent then I just modify a exiting theme that has the same layout, because it is faster, but mostly I like to do it myself. After a few sites you get better and faster and learn more and more tricks.

I would focus on learning the basic's of a CSS layouts before tackling drupal themes. Learn the difference between Classes, ID's and elements, and I would avoid inline styles as much as possible.

Here's some info I used:
http://www.sitepoint.com/article/websites-tables-parts-1-2
http://www.bigbaer.com/css_tutorials/css.image.text.wrap.htm
http://www.w3.org/TR/2001/WD-css3-multicol-20010118/
http://www.westciv.com/style_master/academy/css_tutorial/index.html
http://www.positioniseverything.net/articles/onetruelayout/anyorder (advanced stuff)

css list menus:
http://www.aplus.co.yu/adxmenu/intro/ (see examples)
http://www.alistapart.com/articles/dropdowns/
http://css.maxdesign.com.au/listamatic/

You also need to learn to set CSS for specific browsers. The most irritating browser for me is IE. To pass specific css to IE I us the IE Conditional Comments. (http://msdn.microsoft.com/workshop/author/dhtml/overview/ccomment_ovw.asp). This for me keeps things cleaner.
As long as the IE CSS is after the other CSS it should override it.

<!--[if IE]>
<style type="text/css" media="screen, tv, projection">
@import "themes/.../style_IE.css";
</style>
<![endif]-->

You should be accustomed to build table based sites form PSD files. Once you are more comfortable with CSS you should find going from PSD to css much easier. Then it's just a short copy and past of php code and you should have a drupal theme. Once you get to this point you can look at more advanced themeing like overriding theme function or admin themeing. (http://drupal.org/node/55944)

There is no fast and easy way. It is just a matter of your web development knowledge :-/.