In the Unofficial SEO Blog Navneet Kaushal shows Adam Lasnik's view of supplemental results in Google.
Some people had thought that Google was penalising pages by moving them to the supplemental results index. So that they disappeared from the main search results.
Adam suggests that Google is not penalising sites but moving those that don't have many links to the supplemental index.
Personally I'd rather stay in the main index please! But it does come down to getting those all important links from "good" sites.
I'll discuss what "good sites" means in due course. If people show an interest in the subject.
The bottom line? Get more links into your web site!