This is the famous IE PNG Transparency Bug. Even after IE8 started supporting transparent PNG's it does so only for "100% transparency" and not for partially transparent images (like shadows), thus beating the whole purpose of using PNG's at all (since GIF's also support 100% or complete transparency).
IE8 now shows a 1px border around all the areas with partial transparency and makes them look "rough around the edges" as the previous poster described - meaning they become jagged ans ugly as hell. There is a solution for this bug (and one has to spend hours searching for it online), but it has to be implemented as a span on every single image.
Here is the link to the fix: http://www.optimalworks.net/blog/2008/web-development/ie-png-filter-problems ... It is the "ghetto solution" mentioned in the second comment to this post and IMHO it is worth its weight in gold (how much does a blog post weight anyway). Unfortunately I don't have the time for doing this, only to start over once a new version of the plugin is available...
So my request for Joost would be, if he could implement this solution to all Socialbe icons by default (or possibly to turned on by a checkbox in the option screen)! It would be much appreciated! Thank you!