Multiple tabs, or multiple "browsers" open? I've been using Win 7 for about 8 months and have never seen tab previews on the task bar. However, when you hover over any of the icons, it will show whatever is open for that program. For example, I'm using Chrome now- writing this and chatting w/ my wife (in a popout window). When I hover over the Chrome icon, it shows this page and the chat window. In the same way, if I have multiple spreadsheets open they all show up when I hover over the Excel icon. It does not, however, show individual tabs for each spreadsheet. Just a view of whatever's on the screen. FWIW, I've been digging Win 7.
Perhaps, when you saw it before, it wasn't tabs but different open browsers.