Not being to able to see the category numbers without jumping through hoops was driving me nuts. so here's what I did:
in the wpsc-admin directory, in the file display.group.page.php, find this line:
echo " <a href='#' onclick='fillcategoryform(".$category['id'].");return false;'>".TXT_WPSC_EDIT."</a>\n\r";
and change to
echo " <a href='#' onclick='fillcategoryform(".$category['id'].");return false;'>".TXT_WPSC_EDIT."</a> - " . $category['id']. "\n\r";
Once you do that, it'll print a dash followed by the category id after the edit link on the wpsc "categories" page. I you could tweak the code a bit to put it right after the category name, but this was just a quick fix to show the number somewhere other than via a hover.