Thank you kh99. I think I get what you are after.
Do you know what the variable is that I need to get the current page URL on vb3? On vb4, it looks like its "{vb:raw relpath}"
--------------- Added [DATE]1352216251[/DATE] at [TIME]1352216251[/TIME] ---------------
Maybe "$threadinfo[threadid]" would do it when they access the registration overlay from a showthread.php page.
--------------- Added [DATE]1352218072[/DATE] at [TIME]1352218072[/TIME] ---------------
Sorry, nevermind. On my custom overlay, I had defined the following url variable statically, once I removed that line, the redirect back to the source URL worked normally.