Very strange problem with one of my pages

Hi everybody. I've never paid much attention to SEO before - I figured that if I just concentrated on creating a good user experience then everything would take care of itself. That mostly worked, our home page and flagship product pages have PR 7, and our site turns up for a lot of good search terms.

But.... the flagship product page itself (one of the PR7 pages) is nowhere to be found in the directory. All the other pages show up if I specify the search well enough, but this particular page seems to be banished. If I search for the exact words in the title, I get the "print" version of the same page (PR6) - a page with most of the same text, no navigation, and only 3 backlinks.

Also, a text-ad we ran last month that re-directs to the page in question _does_ show up in the search. Looking at the cache of the text-ad's bounce page shows an old version of our page. It's as if the bounce page for the ad. has somehow replaced our own page.

Any idea what's going on?

[One peculiarity of our server that might be related - surfing to old or non-existant pages will display the same content as the 'closest' real page rather than redirecting or giving a 404. Could that lead to some pages being punished?]

Ok, I'm convinced now that the problem is the way my server was set up:

Links like:
/products/OurWidget/invalid_stuff

where set up to show the same content as:
/products/OurWidget

Not too bad, but following a relative link on the invalid page would take you deeper into the "non-existent" sub-tree. You could recurse indefinately and keep getting the same page.

To google this must have looked like spam, when in fact it was just a quirk of the site code that we never bothered fixing (seemed harmless at the time).

So now that I've fixed it, how long will the page remain punished?