You should be pointing the Roku to  
http://www.tvbydemand.com/1170.ism/manifest (or whatever your ism is named).  You'll also need to specify the StreamFormat as "ism".  The Roku defaults to "mp4" which is why that works without a StreamFormat specified.
Alternatively, you could provide multiple MP4 bitrates, and the Roku will select the one most appropriate for the current network conditions.  It will rebuffer when it switches, but it will only ever switch down, so that shouldn't happen after the first rebuffer.