Andy Beal posted a nice 1 minute answer from Matt Cutts regarding the supplemental index.
There’s no news in the first 35 seconds or so, just a recap of what we already know.
1) Get more links to pages to get them out of the Supplemental Index
2) A site with a given PageRank can only hold so many pages in the real index, get more links, get more PageRank, get more pages in the regular index.
3) The abligitory supplemental is not a penalty.
The video was taken with a 3 year old cell phone, and it cuts in an out, but if you slide the slider till its just under the “M” in Marketing Pilgrim Tv, you may hear something new.
We parse pages and index pages differently when they are in the supplemental index.
He then goes on to explain that the way the supplemental page is stored in the database is different, they have basically a compressed summary that may not contain all the words and phrase relationships stored. So they will not show up in the search results as a normal page would because its not even accessible in their infrastructure the same way.
So while being Supplemental may not be a penalty, it sure doesn’t mean the site isn’t suffering from something. As with anything with Google, get more links, and every thing will be all right.
Update 6/7/07 - Halfdeck posts his analysis of the film clip, it’s worth the read on hiss explanation of Supplemental Results.