So I arrived at the Castle Volkihar for the "Bloodline" quest, the watchman goes "Open the gate!" and well.. the gates don't open. Any suggestions on how to resolve this?
(Yes, I have tried reloading saves a few times.)

Serana is supposed to come up and talk to you before you go in. Did she do this for you?

I also had the same experience. After talking to Serena, the guard had said for someone to open the gate. After multiple attempts at re logging and reloading, the gate would still not open. Am I missing something like a lever or were the gates to open automatically?

Hmm, I'm not sure then. I haven't heard of this particular bug.

Yea she talked to me before the man went "hurr open da dor!".

The gate opens on its own after Serana talks to you so it may be a bug. When you reloaded the previous save was it the autosave from where you got out of the boat? Exiting the area may reset it or try starting a conversation with Serana again.

I have loaded from self saves, exiting the area, and engaging Serena, but the gate has remained closed. I have attempted this with much older saves when I was exploring before reaching the boat for the castle. Nothing seems to of worked.

Yea, I have concluded this is probably a bug with some saves, I used my old character and the DLC worked fine.

I would still like to correct this issue on my character, seeing as that it is my only one.

I have also experienced this bug . Unfortunately for me, my character has no older saves that are before the bloodlines quest(except for one that is 40 hours before)

I really hope Bethesda fixes this aswell. I kind of want to use that other character that has this problem.

anymore progress on this issue? im still having the problem.

If anyone is still lurking in this thread, Nick Breckon of Bethesda Softworks asked if I could send him my saves. If it all goes accordingly, Bethesda should be able to fix the bug!

The gate opened for me but I can't get into the actual castle.

I had this same bug and found a solution. It involves using a glitch (lol) by going through the wall. Get a platter or a plate (platter works the best) and put it up against the gate at eye level. Run straight into the platter and you will start glitching through the gate. It might take a few tries, but I could replicate this exploit and was able to enter the castle and continue the main quest.

Edit: Here is a video on how to do the glitch http://www.youtube.com/watch?v=tDJRKLRtYOY&feature=related - Just do this at the gate and you will get through, and you can go in the door and continue the quest. Don't worry about Serana not getting through because she will automatically appear when you go inside the castle.