Setting base_url in a multidomain setup?
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