Google rolled out a new and improved forum for Places. That would be great and everything, but they decided to completely remove the old forum rather than migrate the old posts to the new system. This is a huge loss to the local SEO community. That forum is full of years of valuable information on Google Places. It's a shame that Google didn't preserve it.
Fortunately, Mike Blumenthal tried to make a copy of it before it disappeared forever. He wasn't able to get everything, but he did salvage a large portion of it. Three cheers for Professor Maps! :)
Here's a link to the Archived Old Google Places Forum.
You can read Mike's post about this mess on his blog, here.