This bug still exists in 3.1 and WebTechGlobal is trying to establish why mass creation might cause it over manually creating categories using the administration interface. Here is a screen shot to explain what is happening quickly…
As you can see, the categories show in the “Parent” menu but not in the standard category list on the right. When I delete the three categories on the right, the rest become visible. I can see no issues with the data, taxonomy matches fine.
I opened a forum post on the WordPress.org website and also found a WordPress Trac regarding this issue.
This problem is effecting Easy CSV Importer and other similar plugins not created by myself or WebTechGlobal. A solution is highly valuable and if you find it somewhere on the WordPress site please provide the link for £15 reward paid via Paypal.