I'm pretty sure you can do this actually, here is an example:
http://www.1stforpartysupplies.com/xcart/customer/home.php
If you click My Account at the top, you get a registration page if not logged in and your profile page if you aren't. If you search the forums you can probably find the code to do this. (My partner did this one so I don't know off the top of my head)
Also, there is a retrieve password page included under the "help" menu, unless that isn't what you are looking for?