Ok I have figured this out - the equivalent for opera and firefox is:
filter: url(drop-shadow.svg#drop-shadow);
where drop-shadow.svg looks like this:
<svg height="0" xmlns="http://www.w3.org/2000/svg">
<filter id="drop-shadow">
<feGaussianBlur in="SourceAlpha" stdDeviation="2.2"/>
<feOffset dx="1" dy="4" result="offsetblur"/>
<feFlood flood-color="rgba(0,0,0,0.3)"/>
<feComposite in2="offsetblur" operator="in"/>
<feMerge>
<feMergeNode/>
<feMergeNode in="SourceGraphic"/>
</feMerge>
</filter>
</svg>
IE being so crap doesn't support the svg values feOffset, feFlood or feMerge so doesn't currently have an equivelant
I'll leave this open in-case anyone figures out how to do this effect for IE
UPDATE
Thanks to psdie for finding this post
IE Version:
-ms-filter: "progid:DXImageTransform.Microsoft.Dropshadow(OffX=0, OffY=1, Color='#444')";
filter: "progid:DXImageTransform.Microsoft.Dropshadow(OffX=0, OffY=1, Color='#444')";
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