Hi Clive Hoggar,
I am happy you like our RadFormDecorator. Now straight on your question:
It seems to be an issue on our side - we have forgotten to remove the white backgorund from the corners of that skin, and we will certainly fix it. In the meantime, you may use the image sprite for the decorated buttons as an external resource. I have removed the white corners for you, and I believe the solution is a good one. Here is the CSS:
background-image: url('ButtonSprites.gif') !important;
.RadForm_Default.rfdButton a.rfdSkinnedButton *
_background-image: url('ButtonSprites.gif') !important;
The sprite itself is attached to the mesage. Make sure the url in the CSS is correct in your project, and do not remove the !important flag, otherwise the sprite will load from the assembly, not externally. The "_" background property in the second selector is for IE6.
the Telerik team