That's a good question.
Let's use the T-Mobile G1 as an example.
Google make a new version of their OS (1.6. for example). They then give it to the various carriers/mobile phone networks. The carriers then customise it, if needed (and that's an important point), to include various things like the T-Mobile G1 boot screen, My Faves, T-Zones, or any other T-Mobile specific app. They might add a bunch of backgrounds, ringtones, etc. If it's a heavily customised version (like phones with Rachel like the Samsung Xperia X10 or phones with the Sense UI), updates will likely be delayed by months to years. When it's ready, they then push it put over the air to various handsets a few at a time.
With the Nexus One on a specific carrier (I forget which networks you could get it discounted on), the same process will apply, assuming they need to customise it at all.
With the SIM free (i.e. not discounted or tied to a specific carrier) Nexus One, I imagine updates will come out very fast. They don't have any carrier specific or custom UI customisations. You'll be able to download the update over WiFi (most updates come over the mobile network OTA so they're done a few at a time), and as Google have massive server bandwidth, it may allow them to push the update out in hours, rather than gradually over weeks.
So, my guess is SIM free (undiscounted) Nexus One owners will get OS updates sooner (a month or so sooner than anyone else), and everyone will get it at around the same time.
However, it might be a bad idea for Google to update all the Nexus Ones on the same day. As with any update, a few people have problems (apps no being compatible and stopping the phone booting, failed updates, bricked phones, etc). Imagine the number of support requests they'd get if they updates all the phones on the same day!
