The div container method described in How to Get Rounded Corners on an iFrame using Border-Radius CSS works for me.
And to do this you simply wrap the iFrame in div tags and give the div these css properties:
The border-radius should be set to whatever you want the roundness to be, and the width must be set to the width of the iFrame, else you will get only a few (if any) rounded corners. But the most important thing is the overflow: hidden, as this hides the iFrames real corners.