Greets:
I just took a look at the Sociable plugin and got it working. (At least it works with Digg and it works in WPMU.)
I would install it within the plugins subdirectory and let folks deside if they want to use it or not.
One major edit is required else it breaks your site. Open up /sociable/sociable.php for editing. Do a search for 'rss-functions.php' within that file. Rename that bit to 'rss.php' and save.
Hope this helps,
-drmike