setzer - 9:51 pm on Jul 8, 2010 (gmt 0)
Awhile back, our forums did drop from the SERPs after a sidewide code change. This affected around 12,000 pages. After a week rankings picked up and things went back to normal though. I suppose Google's crawlers need time to adjust to any major changes, especially ones that impact navigational structure.
I'd wager a guess that the time to "rebound" may be dependent on certain factors, such as the amount of inbound links pointing to your site, whether or not it is an authority in your niche, etc.