For whatever reason, PIE won't work unless you remove !important
from the background
rule.
Though I haven't seen this mentioned in the documentation it worked for me when faced with an almost identical issue.
.wpdm-metro {
behavior:url(/border-radius.htc);
-moz-border-radius:8px;
-webkit-border-radius:8px;
-khtml-border-radius:8px;
border-radius:8px;
margin: 0 5px 7px 0;
height:120px;
width:45%;
background:#66cc33;
color:white;
}