There’s no huge need to upgrade from 3.0.1 to 3.0.2 if you already run WordPress 2.1, but it won’t do any harm either. That said, if you find something that appears to be a bug, the first thing I’m usually going to ask you to do is to upgrade to the latest version. :)

@Martins

Everything should work (it does for me, anyway). To add an article to an existing series:

1. Create your new post, or edit an existing one
2. Select the series you want to add to from the drop-down list (the one that starts out saying “— New Series —”)
3. Select where you want to add the series (”Start” or “End”)
4. Save

If you would like to re-order the series, you can pick which post the current post should come after using the drop-down box under the appropriate series name. Note that you must save the post to a series before you can re-order it.

If these instructions don’t work for you, please file a bug so that I can collect/track all the pertinent information as to why this problem happens.