Known issue yes, fix yes, do I remember the fix...no

but there should be a couple topics out there that tell you how to fix it.
lol - alright, I'll search around - and thanks for letting me know.
And here's the thread that helped me apply the fix: http://www.gamesas.com/topic/1401226-loading-dawnguardesm-in-the-creation-kit-merged/page__p__21336689__hl__dawnguard%20crash__fromsearch__1#entry21336689
Incomprehensible to me for the most part, but toward the end it's explained more simply:
Go to your C or D drive - wherever Steam is installed (for me it is D), and navigate as follows: D: --> Program Files(x86) --> Steam --> steamapps --> common --> skyrim. Look for an ini file named SkyrimEditor. I always make a copy of the file and drop it on my desktop, just in case I mess up or it doesn't work, or whatever. Now open the file that's in the skyrim folder. In the document, scroll all the way to the bottom and then up just a bit until you see the heading, [Archive]. Under this heading, change the last line from:
SResourceArchiveList2=Skyrim - Shaders.bsa, Update.bsa
...to:
SResourceArchiveList2=Skyrim - Shaders.bsa, Update.bsa, Dawnguard.bsa
...and then save and exit.
And there you have it.