Testing out 3.5 Beta 1. Noticed something strange under Multisite. If I'm on the primary site and output the wp_upload_dir() returned array I note the basedir items is properly set to the '.../wp-content/uploads' path. Then if I call switch_to_blog(3) then call wp_upload_dir() again the basedir does not change.
However if I'm on a subsite (not the main site) and call switch_to_blog() to some other subsite the basedir does properly update.
In WP 3.4.2 the basedir item would change to the proper blog media upload path.