Valid values for streamFormat are called out in section 3.3 of the Component Reference.
A quick HEAD request on your URL tells me it's not compatible. The URL points to an XML file that the Roku doesn't know what to do with.
curl --head "http://revsam.org/revsamtv.wvx"
HTTP/1.1 200 OK
Date: Tue, 29 Nov 2011 16:27:10 GMT
Server: Apache/2.2.3 (CentOS)
Last-Modified: Thu, 25 Mar 2010 05:36:20 GMT
ETag: "5e47fb-66-6d2b3d00"
Accept-Ranges: bytes
Content-Length: 102
Connection: close
Content-Type: video/x-ms-wvx
And looking at the actual XML shows it's a Windows Media redirector file that points to an MMS stream which is not compatible.
curl "http://revsam.org/revsamtv.wvx"
<ASX version="3.0">
<Entry>
<ref HREF="mms://revsam3.chennaistream.net/revsamtv"/>
</Entry>
</ASX>