From what I understand of the OTA firmware update process, it goes a little something like this...
When your device connect to the network, it logs in to a software repository held by Google in the background, which contains all the files needed for that firmware version. If any changes are made in the repository, the device will prompt the user that updates are available, and when the prompt is accepted, those new files are downloaded, the old ones over-written, and the firmware updated. The exact folder that it checks is based on the current build of the device (firmware), and the manufacturer and model of the handset.
When Google sign off on a new firmware release, it first gets uploaded to the folder that the Developer Release handsets check. This allows anyone with testing handsets to get an early peek at the firmware, run some tests and make sure everything works. All manufacturers will have some of these test devices for their own purposes.
When the manufacturers are happy that the firmware doesn't break their hardware, they send their approval off to Google, who will then upload it to their relevant folder. Google may also, at their discretion, decide to phase those releases so that their infrastructure is not overloaded by all Android devices downloading the updates at once. In this way, the manufacturer does have some input into the firmware (for example, on the HTC (not with Google) devices, they may want to push out their own additional features or updates not covered by the standard Google package), but it is all ultimately pushed out by Google at the end of the day.
Since which updates are received is down entirely to the build number, make and model of the device, it is network independant, and you will continue to receive updates as long as Google and HTC keep churning them out.
