With IE7 expected to be pushed down to PCs via Windows Update, the urgency to ensure that your websites work correctly under this new version has just increased ten-fold. The expectation is that this won't be a choice but that it will be forced onto most PCs via Microsoft's Windows Update mechanism.
I'm testing my sites now with the recent IE7 Release Candidate 1 which can be downloaded at Microsoft's website. Since I didn't want Internet Explorer 6 to be upgraded on my dev box, I wanted to be able to run IE 7 standalone. The link below will provide the tools to run IE7 in standalone mode.
This is Jon Gallowy's updated version for IE7 RC1. The old script doesn't work properly and this script applies a hotfix to allow the tabs to work in standalone mode.
I'm currently using it and its running perfectly. The readme.txt file included in the tool will explain how to install IE7 in standalone mode so DO NOT install IE7 before reading that file. Otherwise, it will update IE6 to IE7.
Definitely consider using this especially with IE7 just around the corner.