I have a cable box from Comcast with HDMI output and I cannot get a picture working on my Sharp LC-26DA5U through its hdmi port. Am I forgetting to do something? I have the box set up to do 16:9 720p.

1) has the HDMI output been enabled by the cable provider?
2) does your TV "accept" 720p video input
3) try setting to 1080i
4) any error messages? what are the symptoms/results?

1) The HDMI Output has to be enabled?
2) Yes, it does.
3) I can try that
4) No sound and no picture.

1) providers have been known to not enable HDMI, firewire, USB, etc on STB's... had to ask.
4) unplug both the TV and STB from power (remove cord from wall). Power up TV, then STB. May just a an HDCP "handshake" issue.

I just got my HD cable box from Charter (Scientific Atlantic 8300HD).
The manual states that one should not hook up your cable box with an HDMI cable unless the TV supports HDCP.
If the TV does not support HDCP, then the outcome is just what you are getting (not audio, no video)

So I would recommend checking that your TV supports HDCP

99.9% of TV's with HDMI are HDCP compliant. Although... there have been some "compatbility" issues reported about some TV's and STB's.

I just checked sharps manual online for my tv and mentions nothing about hdcp. I then checked one of the46/52" models and it says hdcp. I guess maybe mine doesn't support hdcp.