To answer the original poster...
As you noted, there are variations on each version of Android that are mobile carrier specific. What happens is that Google hand over a given standard version of Android to the carrier, and they may choose to customise it, test it, and roll it out over the air. This is actually slightly incorrect, as it implies the carriers push the update to your phone. This isn't what happens.
Android runs a process called the Checkin service, which checks for updates for the OS every day. As a side note, it also checks for updates to your applications at the same time. The version of android that is currently installed has some information attached to it which is unique to that version. From this information, the Checkin service can look for an update that is correct for your phone. So, if your version of Android is version 1.6, for the G1, customised for T-Mobile UK, then the Checkin service will only inform you about updates specifically for that version of Android. Even though 2.0 and 2.1 is available for other handsets, you won't be informed about them on your phone.
You said you're getting an "unlocked" phone. Has this phone always been unlocked? Or was it tied to a specific network at one point? The difference here is that unlocking a phone that is currently tied to a given network doesn't change which updates you can get - you'd still only be able to get updates specific to that phone on that network. If the phone has been unlocked from the start (it has never been tied to a network, and is brand new), then it will have a non-carrier specific version of Android on it, and you will get informed of updates as and when they're released for that phone (usually before all the carrier-specific ones, as your version won't need to be customised by some carrier).
There are also other ways to update. Some manufacturers (HTC for example) put their updates on their website. You can install them by simply copying the file to your SD card, and holding down various buttons when you start your phone and pressing various keys to install it. Again, it will only allow you to install the correct version for your phone. So if you have a T-Mobile customised version of Android on your phone, and try and install the HTC version of Android, it'll not let you and give you an error. In this sense, whether your phone finds the update online itself, or you download it manually, it has the same restrictions.
Finally, you can "root" your phone, which means you get super-user / admin access. You can then bypass the security feature on your phone which normally prevents you installing any version of Android, and install a customised version (or in fact any one that's suitable for your model of phone). There are some minor pros (speed, additional features, more memory) and pretty big cons (invalidated warranty, risk of rendering your phone completely unresponsive, less protection against malicious apps that can then run in super-user mode) to this, so I'd advise reading up on this before trying it.
Finally, the Nexus One is way better than the Motorola Milestone (aka Droid), unless you absolutely must have a keyboard. The Milestone is currently the best Android phone with a keyboard. You may also want to consider the HTC Desire, which has an almost identical spec to the Nexus One, but has a few minor things better (optical trackball, quad band, Sense UI) and a few minor things that aren't as good, like the Nexus One's brilliant noise cancelling microphone and voice search.