Bill Barnett has posted a couple of great articles detailing the new APIs we've added in SP1. Check these out to see the powerful extensibility points for controlling load patterns in a load test, and more/better hooks in web tests that enable some key scenarios such as easily filtering some types of dependents and easily capturing page load times via the API.
Since the beta, we added support for IE8. The interesting thing about IE8 is that by default it opens 6 connections to the web server, rather than 2 which is used by previous browsers. FireFox 3 also uses 6 concurrent connections. While this makes playback of a single test faster, it doesn't actually increase throughput in a load test.
As I said before, and Bill details on his blog, the biggest thing to be aware of with SP1 if you are running load tests on an agent is that agent, controller, and VS clients must all be upgraded at the same time. So you can't run SP1 VS against an RTM controller and agents (and visa-versa).
We've also fixed a few additional bugs since the beta (all of these were in the RTM version of 2008, none were regressions in the beta):
Team Level 1
Test Infrastructure Team
VSTS failed to install on top of VS Pro + SP1
Load Test Team
Web tests do not accurately emulate and measure the page response time using IE8
Multi-agent -total user load and TestRunning are not calculated correctly
Dialog clipping - the "Disable background discovery of test methods" string clips on localized versions.
When a test file (test storage) has dependent files, we do not enumerate tests from it
WebTest: No way for Web test code to get the ContentLength of a request
Recorder: Low level recorder isn't working for multiline textboxes
Web test hidden field correlation feature doesn't work for Ajax update panels
Web test execution: file uploads are posted twice when uploading to sites that require NTLM authentication
Unit Test Team
Unit Test inheritance: user experience running unit tests with inheritance from command line is broken (mstest reports an error)
Short of finding a catastrophic error and clearing up one last problem we are seeing with applying the patch, we are now done with SP1 and making great headway on our next release.