I haven't been able to upgrade to CS4 yet, however I want to make certain I have the most "up-to-date" Camera Raw plugin for CS3. I am currently running version 188.8.131.52. Is there a higher version of the plugin I could download and use?
Go to http://www.adobe.com/downloads/updates/ and change the dropdown at the top to say Camera RAW - Windows or Mac and Go and then you will see all the updates for ACR on your platform.
CS5 should be out in a few weeks or months, so save up for that assuming you're on a PC or your Mac is new enough, otherwise you'll need to get CS4 while it is still available because it is the last version compatible with non-Intel Macs.
The short answer to your question is, "No, there is not a newer version of Camera Raw for Photoshop CS3." And there will not be a newer version. This does not, however, "force" you to upgrade. You can use the latest DNG converter create Digital negative copies of your images, and those can be edited with the version of Camera Raw that you are using.