We do have a Slingbox with an HDMI input in the Slingbox 500. That said, relying on that to get your video signal has the risk of you running into copy protection from the cable box. This article covers the issue:
If your cable box has HDMI only, we recommend the use of an HDMI-to-component converter to help hook the Slingbox to this cable box. Here is some information about that:
Hope this helps!