[Original answer at http://acs.codeplex.com/discussions/352424%5D
The ACS WordPress plugin does not directly support multisite WordPress installations today.
There are several ways this could be solved, but the easiest is probably to set a cookie with the desired return URL when a user clicks on one of the login page buttons, and then have WordPress redirect to that URL after it processes the token received from ACS. The redirect behavior post-login is actually defined in the WordPress site code (wp-login.php) and not the ACS plugin (which just overrides the wp_authenticate function and has some code for rendering the login page).
