You may want to take a look to Classification Web Service. You can use it (see 'setCategories') to apply some categories to some nodes (identified by the predicate object)…
I had a look at setCategories and for what I understand I need to have the category already existing in Alfresco so that I can obtain its path or UUID and then assign it to the content. What I need is to create a totally new category from the WebService.
I have followed this (very useful) example but also assumes that we have already the categories
It seems that that feature is scheduled for a future release… at least, according to the wiki… =/
I'm not an Alfresco developer, so I can't tell if it's already implemented in Alfresco 2.0 (that page was updated for last time on November 2006…) Try to use the method described there… if repository doesn't recognize it… I assume that creating a category is not available yet =(
Maybe an Alfresco developer could tell you more… sorry.