Nice looking plugin. When there are available so many corner ribbon plugins for particular causes, it is good to have one that can be customized like this. Seems to work OK with WP 3.4.1.
But I see one small issue with the appearance (as at http://italiapa.com/). There is a jagged black edge along the side of the ribbon. I guess this is because your process to generate the PNG leaves some pixels not quite black, and so not quite transparent. I don't know how you can get round that - maybe by disabling any kind of compression when making the PNG.
It would also be nice to be able to select the font and font weight for the text. My first version of my ribbon had pale text on a dark background, which avoided the jagged line problem, but the font weight was too light for the text to show up clearly.
I know I can work round these issues by creating my own image, and I may do that when I find the time, but they would be better fixed in the plugin.