I'm in exactly the same boat. Wordpress install running fine for over two years ... Then suddenly this morning (about 8am EST I think) I got the same error you described.
Here's what I've discovered: When the error starts happening, somehow, the "/wp-content/cache" directory is no longer a directory. But there's now a file called "cache" in the /wp-content directory ...
Delete the file, manually recreate the directory, and everything starts working again. But only for a few hours ... Then the folder-changes-to-file issue happens again.
The fact that we both started experiencing the same issue on the same day sure looks like more than a coincidence.