WordPress / mod_rewrite expert needed

[meeble]

Hello,

I am trying to use mod_rewrite and modify WordPress' URL handling to allow for passing variables to PHP from the URL while maintaining "pretty permalink" functionality. (on WP 3.2.1)

Something like this:

http://epiclasers.com/hello-world
(normal with permalinks set to /%postname%)

http://epiclasers.com/state/hello-world
(WP should display the same hello-world post as above, but also would pass the state string to PHP as a variable)

http://epiclasers.com/state/city/hello-world
(WP should display the hello-world post, but would pass the state and city strings to PHP as variables)

I added the following mod_rewrite code to my .htaccess file:

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(\w+)/(\w+)/ /index.php?st=$1&city=$2 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(\w+)/ /index.php?st=$1 [L]

which kind of works. The variables are being passed, and PHP can grab them with:
$state = $_GET['st'];
$city = $_GET['city'];

The problem is that WP is getting confused by the URL and isn't finding the post. URL's like these:

http://epiclasers.com/state/city/hello-world
http://epiclasers.com/state/

are bringing up a page not found error. Also, when entering this URL:

http://epiclasers.com/state/hello-world

It redirects to: http://epiclasers.com/hello-world
which then displays the post fine.

How do I modify the WP URL handling code to ignore the /state and /city parts of the URL without also breaking URLs like:

http://epiclasers.com/category/category1
http://epiclasers.com/tag/tag1
http://epiclasers.com/category/category2/subcategory

etc.

Thanks,
Devin