I assume someone somewhere has made this connection before, but I've never run across it. But it's so obvious, now that I know. Like having a jigsaw puzzle where the pieces don't fit, if you bang away at them with a hammer long enough, they do fit! Granted most of my King Arthur recollection is from Excalibur and Wikepedia (that famous King Arthur book)
Kara Thrace is Parcival. She leads them to the false grail. She redeems herself and leads them to the true grail. She doesn't start as a knight, but is knighted when no one else comes forward.
After this, I'm stretching things.
Adama/Roslin are Arthur. Like it would be Billy or someone. It's Adama earlier on and Roslin later on. Gets ill as things get worse. Sends the knights off to find the Grail. More hammer needed here.
Mordred. Cavil Turns on his father/creator. Main bad guy. Can only be killed by his own gun/sword???? Also there was incest in there somewhere. Didn't he sleep with Ellen?
Lancelot. Tigh/Ellen Comes from across the
Baltar Um, which knight gave the enemy access to the armor codes? Possibly Merlin, since I don't know who else would be Merlin. He does advise the king, gets betrayed by Morganna.
Guinevere Um... Whichever of Adama/Roslin isn't being king and is sleeping with Lancelot? (oh please let it be Roslin/Ellen, er.. did I type that out loud?)
The grail is obviously Earth. There's even a false one. There's also the fall of a great empire prior to the story.
I'm sure that there are other parallels, since what I've got so far is pretty thin.
I'm sure that there are other obvious parallels that can be seen after being woken prematurely by a thunderstorm. That's why I have literary friends. Of course, there's probably a reason no one came up with this parallel before, too.