Zero Intrusion. This is why I fully support WP7 model. OEM and Carrier can pre-install apps, but, they can be removed just like any other downloaded apps. This is also why you don't have that infamous spyware found in some iPhones and Androids. The OS itself is in complete lock down.
They do however, able to include their Shop Link in the Marketplace Hub. And AT&T can include default "pay to phone bill" if you want to buy an app. And you cannot remove that payment method, only adding your own credit cards. I am not sure if they have parental control on purchases. So, it can be a problem if you buy the phone to your irresponsible kids.
And they can change Bing search to other search. (thus, double check that before purchase). And if you are in China, different Map/Search are used.
I praise WP7 a lot, because the newer safer model, such as single tasking, OS lock down, are the golden rules to prevent anything from ruin your WP experiences. It is one major rule to keep OEM from loading their own strange GUI. And there is no virus/spyware that is meaningful due to lack of multi-tasking on 3rd party apps.
(one good thing from carrier is warranty. In any accidents, you can replace your phone within a year.)