If you mean the Safari thumbnail view - that is not possible with iOS 5. Safari now uses tabs to pen new web pages. Tap on the + sign on the right side to open a new tab. Other browsers still use the thumbnail view for open web pages. iCab Mobile has the feature.
If you are asking about the bookmarks bar in Safari - go to Settings>Safari>Always Show Bookmarks Bar>On.
If that is not what you are trying to do, post back with more details.