By default, IE9 will show 2 rows of your favorite sites on the about:tabs homepage. However, you can customize IE9 to show up to 5 rows with a simple registry modification.
Inside the NewTabPage registry key, create a REG_DWORD named NumRows, with a value of 2 to 5.
Or, simply merge one of the following registry scripts:
You'll need to restart IE in order to see the change.
Hope you're enjoying the IE9 Beta!
Update: In the comments, I mentioned that "If a HTTPS-delivered page is delivered with the headers Cache-Control: no-cache, or Cache-Control: no-store, or Pragma: no-cache, IE will not add the page to [...] NTP. Sadly, there are some big important sites that are delivered with such headers (e.g. GMail, IIRC)." IE9 Release Candidate relaxes that restriction and such sites are eligible to appear in the New Tab Page unless the user sets the "Do not save encrypted pages to disk" option inside Tools > Internet Options > Advanced.