tedster - 7:03 am on Aug 14, 2010 (gmt 0)
I think it could be. The home page is not a new version of the old URL - that is, the content has not "moved", it's just gone. I know of several websites that got into ranking trouble by 301 redirecting many pages to home (or a top level page of some kind) instead of returning a 404 or 410.
It doesn't pay to squeeze on the PR too tightly. I'd say look for important backlinks that point to problematic pages and create appropriate content at the same URL - even if it's just an explanation about the change to the website.