e911 - 12:01 am on May 9, 2008 (gmt 0)
Same thing happened to me. I have a popular authority site that dates back to 98 and they found hidden texts on old unused pages that were sitting on my server. They sent me the same e-mail and I started to go through the site to fix everything. Google actually put my site back in the serps with all the original top rankings exactly 30 days after I got that e-mail and even before I was done correcting everything (I hadn't even requested reinclusion yet). So if they think your site is a good site, you shouldn't have to worry. The only thing they did was to remove 1 point from my home page google pr, which didn't affect my traffic anyway.