I just made a little tutorial for Acorn 2, titled "How to Fade Out Your Image, and Other Interesting Techniques Using a Duck".
Someone asked the other day how to do some liner transparency effects in Acorn, so here it is. At the end of the tutorial, I also show a technique on how to use layer groups to have local compositing- sort of like cheap layer masks.
I should also note, that for Acorn 2 all the documentation has moved into a VoodooPad document, which has been a lot easier to add to than the Wiki I had before. And you can download that VP document, using a link found at the bottom of all the document pages.