This issue seems to be well documented in a closed ticket on d.o, and in a ticket here on ubercart, and some other comments scattered about, but all would seem to indicate that it is fixed by now, however I still have the problem in ubercart 6--2-0-RC3
* http://drupal.org/node/305177
* http://www.ubercart.org/issue/6105/anonymous_user_loses_cart_contents_af... along
an anon user is prompted to "click here to login with an existing account and return to checkout". and when they do, their cart which previously had some number of products in it, is empty after login.
