Sadly I can confirm that although they call it Sci-Fi HD, they are showing the SD version of Battlestar Gallactica. It is 16x9 shown in a 4x3 window which means there are bars on all 4 sides and the picture is tiny. I can use the "zoom" function to fill the screen but since it is only an SD source, it looks crappy.
Is this a comcast thing, or an issue with the Sci-Fi channel?
Not all of them. My Comcast feed went live on Monday. Last night I checked the season 4 episode shown at 10:00 or 11:00. It was in full screen HD, with Dolby 5.1 audio to boot. I didn't watch the episode since I only wanted to confirm, in my mind, that the season 5 opener would be in true HD.
I have no idea as to why the majority of BSG episodes were letterboxed. I was checking the channel frequently and was getting increasingly anxious until last night.
My experience with Sci-Fi HD on Comcast is that anything new since they've started the HD feed has been in HD, but that's about it. So we'll get Tin Man in HD, but not last year's TV episodes. I'm pretty sure we'll get Battlestar in HD starting Friday, but I tend to be skeptical about anything a year old or more.