There are some commercial modules that don't work in child portals because they log off the users, even after using this patch: http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=567799
I think that, for a COMMERCIAL module, it must be MANDATORY to advise wether or not it can be used in child portals without limitations.
If this shall not happen, i suggest to maintain a free and public list of COMMERCIAL modules that log off users in child portals!!!
I'm just experiencing this problem with a commercial module, and am waiting for the author to solve the problem OR to advise about it before selling.