Hmmmm..It's funny that it happened right after your weekly update.
I would check a number of things before I would blame the "hidden text". That is really rampant throughout the web and google hardly ever really penalizes for it. It has to be blatant spam for it to have an effect and even then it isn't always hit on. I have even reported it and seen nothing happen really.
Have you run all the regular tests. Like xenu linksleuth. Have you checked your raw data in your logs to see if there is a problem for google. Is your robots txt. unchanged? Have you checked to see if your headers are Ok.
Have you even run the site: mysite command to see if you have dropped a bunch of pages. Big Daddy is having a bad effect on a lot of sites.
Tedster- where is your famous list of threads to go thru for sites experiencing problems? It really is good advice.