After Paulo alerted me to the availability of IE8 RC1 (and after waiting a little while for the links to update from Beta 2 to RC1) I had a seamless install experience. No need to uninstall B2 on Vista, just run the installer and it takes care of the upgrade for you.

It’s worth checking the upgrade details on the IE blog here for your particular OS / IE version. You can install IE8 RC1 from the IE website here. Note: this build of IE8 will *not* work with Win7.

Updates in this release according to the IE Blog:

• Platform Complete. The technical community should expect the final IE8 release to behave as the Release Candidate does. The IE8 product is effectively complete and done. We’ll post separately about the thousands of additional test cases we’re contributing to the W3C. We've listened very carefully to feedback from the betas. With the Release Candidate, we’re listening carefully for critical issues.
• Reliability, Performance, and Compatibility improvements. We’ve studied the telemetry feedback about the browser's underlying quality and addressed many issues.
• Security. We’ve worked closely with people in the security community to enable consumer-ready clickjacking protection. Sites can now protect themselves and their users from clickjacking attacks “out of the box,” without impacting compatibility or requiring browser add-ons.  We also made some changes to InPrivate based on feedback from customers and partners.