Hello Jorge,
I am really sorry for the delay of this reply as it is a weekend for us here. Yes, it was intended to be added into version 3 but there's some road blocks that prevented us from allowing the blog post to be posted into multiple categories. The main reason? the access control level on each category. It's kinda confusing because imagine if you create a blog post titled "hello world" and filed it under "category A" and "category B".
In the category settings,
category A - allows public viewing
category B - only allow private viewing
So, if a public user tries to view your blog post, should it use the access control from category A or category B? Although we can apply "deny rule always win", it's going to be really complicated in the future as we can foresee user's asking
1. Why can't public users view my blog post??
2. Why is my private blog post available publicly??
We can already see all these support request examples on Joomla 3.2's ACL and EasyBlog's internal ACL already