This is the third service pack from Microsoft for their soon to be retired operating system. For many (myself and Ken McGuire included), this will bridge us through to a brand new OS. But it won't be the dogs dinner that is Vista.
Both of us are going to complete the switch to Apple - I am partially there with the third machine in the house (after my main PC and laptop) running Leopard. When I have to replace my main one (currently well spec'd and not anywhere close to needing to be culled) I will try and go for a Mac Pro and use Bootcamp to run the Apple OS and XP (as the iMac does now).
However my dependency on Windows and software that only runs on it has almost vanished as I now work more and more on Web services that are hosted remotely and accessed through the browser. Thunderbird is my main desktop client and that is available for Apple.
For those of you who are running two machines (main PC and laptop for example) be sure to install Service Pack 3 on one of them only and ensure that all is working well before you install on the second. It has been a long time since a Service Pack screwed up a computer on me but it can happen and it is messy.