Currrently the user has to choose their category each time they make an entry, the down side to this is you cannot have a "per user" blog easily this way.  For example
if there are 3 categories
user 1
user 2
user 3
then on the users menu  "My blogg Submit" it would look at the UID and set the category "by default action" to that UID NOT to the "default" listed in the category section.