I have two google maps. The first is created using the Google Maps API (v3) and contained in #map1. The second is displayed in an iframe using the suggested embed syntax from Google and is contained in #map2.
Here's the fiddle: http://jsfiddle.net/wmcmeans/mPGWx/7/
Here are the snippets:
<div id="map1" class="gmap left bling"></div>
<iframe id="map2" class="gmap right bling" height="425" width="550" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" src="http://maps.google.com/maps?hl=en&q=augsburg+germany&ie=UTF8&hq=&hnear=Augsburg,+Bavarian+Swabia,+Bavaria,+Germany&t=h&ll=48.451123,10.862346&spn=0.004981,0.00912&z=16&output=embed"></iframe>
This appears to be fixed, so it's no longer an issue. You can verify that a map container's border-radius style is indeed respected, using the v3 map API, at this fiddle.
<div id="map3" class="gmap"></div>