Setting base_url in a multidomain setup? I have functionality that relies on having $base_url set in the settings.php file, but I have a multidomain site in order to keep languages separate. 

The sections of my site are www.mysite.com and www.de.mysite.com. If I set base_url to "www.mysite.com" I get German language content on www.mysite.com.

How do I set the base_url for a multi-domain, multi-lingual setup so that things like my image style generation can work correctly?

This is a single install, not multisite.
I guess you need separate settings.php file for each site.
You can create special folder for each domain in your sites directory and set custom $base_url in each settings.php file.

.
+-- all
¦   +-- modules
¦   +-- README.txt
¦   L-- themes
+-- default
¦   +-- default.settings.php
¦   +-- files
¦   L-- settings.php
L-- de.mysite.com
    L-- settings.php
L-- ru.mysite.com
    L-- settings.php


Documentation about site folders: http://drupalcode.org/project/drupal.git/blob/c5d6e6334fb7a71ecf1dbc7e06a7de8ad9547b27:/sites/default/default.settings.php#l25