This seems to be a trend at Microsoft. The first time I heard of an OS needing an update to allow for a bigger update, like a service pack, was for Vista before SP1 was released. There were two updates that had to be applied first or the service pack wouldn't install.
Don't worry if you don't have a notification yet. Microsoft is releasing it in a staged manner, so it may be a day or so before you get a notice. The notification comes from the carrier, which signifies that they have done all of the necessary testing. The update itself though comes from Microsoft and is installed via the Zune desktop software.
Microsoft has indicated many times in the past that updates on Windows Phone 7 would be far more accessible to the end user than the updates for Windows Mobile were, where 100 percent of the distribution was in the carrier's hands. Democrats and Republicans were more likely to work together than for a carrier to actually provide updates for Windows Mobile phones. If the carrier is still control of the notification, the process that starts the update, it remains to be seen how much better it will be. Hopefully the carriers have seen the light and by providing updates it fosters consumer enthusiasm and loyalty. Just ask any Apple iPhone owner.
My question is, what happens if you buy a sim-unlocked phone from a third party and then use it on a network that doesn't have a Windows Phone 7 partner agreement with Microsoft? Obviously the carrier will not send a notification to the phone to start the update. I'm sure we'll see how that works, or doesn't, in the coming days as this "pre-update" is rolled out.
You can read Microsoft's take on the update here.