I've noticed that it rescans for media every time I touch a "media" file within Astro File Manager..
Ie: just go into Astro, go to SDCard, find a ringtone/mp3/something... and just do a "rename" on it (don't even have to rename the file), then accept the change (even if you didn't change anything)....
I can consistently cause mine to rescan every time I do this procedure.