Well, I can't confirm how the rest of you got the error, but I rec'd it after I had upgraded from 4.6.2 to 4.70. At first everything ran fine, but a month later, I opened the DNN sln file, and did a recompile without thinking. I believe the *source* code was 4.6.2, and because it had overwritten all the dlls for 4.7, the above is the error you will end up receiving.
It's a bit confusing, but I would suggest restoring your backup (you do have one, right? ;-)), and if you have previously downloaded the source, you should download the source for the version you're overwriting, and then continue w/the install. Don't make the mistake I did and use the upgrade, since inevitably you will open the sln file, and compile, and things won't sync up.
Hope this helps some of the above people who had these problems and didn't get any responses to their questions.