So post-exiting the soul cairn in the dawnguard quest line I'm suddenly getting random relatively frequent CTDs where a C++ runtime error box pops up. I have to use task manager to close the game.

Wondering if there's any fix known for this yet or if my save game is FUBAR now and I should just go back to an old save and put the Dawnguard questline on hold until a fix comes out.

There is sort of a fix. It was suggested in one of the main threads on the issue to enter the Soul Cairn at the start of the quest, then leave and then re-enter again. Here's the mega thread: http://www.gamesas.com/topic/1421930-c-runtime-error-mega-thread/page-5?hl=%20verify%20%20integrity. Scroll down to post #136.

That worked for me on a vampire character, but I've just run into the problem again despite trying that trick, on a partially soul trapped character. Seems to me that the Soul Cairn is well and truly hosed in this regard (especially if you want to keep your character human!) and only Beth will be able to fix it (and somehow I soubt that's going to happen). For the time being I'm going to just completely ignore the Dawnguard questline unless I intend to make that character into a vampire.