I would check the installation of your server-side certificate first. You can do this with curl from a command line on your workstation:
curl --cacert cert.pem
https://yourhost.domain.com/yourURL.htmcert.pem is the same file you included in your .pkg at /certs/cert.pem, but copied to the current directory on your workstation. cert.pem must include the CA cert of your web server.
Note that firmware v2.6 and later includes a CA cert bundle in the common: device that includes common Certificate Authorities trusted by FireFox. If you got your server cert from one of these authorities, you can use "common:/certs/ca-bundle.crt" instead of "pkg:/certs/cert.pem".
The Http.AddHeader("X-Roku-Reserved-Dev-Id", "") and Http.InitClientCertificates() have been covered in previous forum post. I'll refer you to that post if you desire implementing this content protection scheme:
http://forums.roku.com/viewtopic.php?f=34&t=27562&p=171752&hilit=SSL#p171752I'm sure you did it already, but your code also has to specify the URL:
http = CreateObject("roUrlTransfer")
http.SetPort(CreateObject("roMessagePort"))
http.SetUrl(url)
http.SetCertificatesFile("pkg:/certs/cert.pem")
response = http.GetToString()