Now with the latest version of WordPress 2.8 feature may ease you to install wordpress theme automatically without hassle. Just select the term that you may need, then click search. There you go..
This feature, may enable you to select the theme criteria such as the number of column, favorite color, fixed or flexible width, custom header, custom option or even customized theme as well.
Just most theme site may offer, you may preview first the theme that you interested. If you liked how it really looks, you can easily install the theme to your wordpress blog.
So by now, we can install both the themes and plugins automatically. who knows what feature they will add to the next version of WordPress.