I recently updated to wp 3 and atahualpa theme to 3.5.3 in a self hosted site.
The problem is, when I try to publish a new post with some categories selected, it marks only the default one and that´s what it shows in the blog. No matter what I do it still marks only one category. Actaully i changed the default name by (News) Novedades.
www.puntodeluna.com
Do I have to delete the DB and begin from zero?

After you create the post can you select more categories?

Hi, After I create the post I can select any of those categories, but when I press the publish button it comes again to the default category.
I try to assign also the categories to many posts as a batch but again it overwrites my selection with the default category.

Does this same behavior occur if you use the 2010 theme?

Hi again, I am using "Atahualpa 3.5.3 por BytesForAll" is it the one you are pointing at?

I activate "minimalism 1.0.2 por Christian Genaehr" to test and the problem is still there.