If you have Windows XP and install SP3 you will be locked into IE7. You will not be able to downgrade to IE6.
Now I’m willing to bet within a few weeks of SP3’s release we’ll see instructions on how to get IE6 up and running. At the same time I don’t see this as an entirely bad thing. IE7 is certainly better than IE6 in terms of standards compliance. But I’m not a fan of locking users in like this.
I wonder, though, why you’re locked in. Is IE7 some how integral to SP3? I thought Microsoft was suppose to separate its browser from its OS. If they’re integrated again that might break certain judgements made in the EU.