Paul Thurrott has MS on the record regarding Windows Phone updates. The end result: MS pushes the updates but the carrier can block it for up to one cycle for testing purposes. They also state that they are working with carriers to prevent delays and presume that if there are delays then the consumers will go ape sh!t and get the carriers to let it go through. By the way, it’s clear that this is regardless of if it’s a small update or a large one (through Zune). AT&T chimed in and said
AT&T has changed its testing processes to match the needs of the market, and the needs of Windows Phone. We plan on continuing to follow the new model going forward with regards to software update. [i.e. will not turn off software updates to force device upgrades.] We will churn things around more quickly than we did in the past. Updates extend user value, and make for happy subscribers. We are jointly incented.
The last bit of info is when the first update is coming and Microsoft corporate vice president Joe Belfiore said "We are shipping a compelling update very, very soon". Good stuff