It doesn't matter if HTC built the Nexus one or not. It's not HTC's device it's a Google device just like the G1 or the MT3G. Thus HTC does not currently have any of it's OWN devices that are operating at above version 1.5
Google supplied it's version of Android 2.1 to HTC to install on these devices, just because HTC has Google's version doesn't mean that they can take it and install in on other devices. They would have to take the open 2.1 source code and develop their own UI from it.
And you are correct, it is not that HTC doesn't have access to 1.6, it's just that they don't want to go through the effort to make it fit on the Dream.
gbd
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