The fact that all of the WinPho7 devices share more or less identical hardware requirements means that all updates should apply to all phones, rather than for instance with Android where one manufacturer may release an update at a particular time.
Not strictly true. This is just down to the fact that MS don't allow OEMs to customise the user interface in the same Google allow with Android. The delay comes from Google having to release an update and then the manufacturers to pick that up and modify their GUI skins to suit it.
(Advantage of having a Nexus S is that I don't need to wait for that to happen, so am already rocking 2.3.3 which came out yesterday)