The Forza team recently launched their online stats for Forza Motorsports. The stats are still a bit rudimentary, and there's no way to log in to get a brief overview of your own statistics a la Bungie's Halo 2 stats. However, there is a GET interface to retrieve gamertag details.
The bar displays the current gamertag with a link to its statistic page, and the tag's ELO, 1st place races, 2nd place races, 3rd place races, and total races. You can refresh your statistics at any time (this currently works best in Firefox with Greasemonkey -- you may have to click Refresh multiple times in IE with Turnabout), check on the statistics of another gamertag (type the tag into the input box and click "Check" to navigate to that tag's stat page), or set your a different gamertag to track (type the tag into the input box and click "Set"). After installation, you'll notice that the bar says "No tag". You need to set a gamertag for it to watch.
Caveat: Like Bungie's Halo 2 statistics, the Forza statistics may occasionally be out of sync with the actual Xbox Live data. This can manifest itself a simply not finding any statistics at all, or finding only partial statistics (ELO, but no race numbers), or it might just not reflect your most recent races (ELO changes, race count increases). You can try Refreshing the stat bar data, or Checking your gamertag's statistic page to see if the data has been synced up yet.
Hopefully a more robust data source will be available for Forza stats (RSS, please!), and hopefully even more statistics will be forthcoming.