HTC will indeed have the stock android source, but they put Sense on it before releasing it. So you can't get a Sense-free Desire ROM from HTC - you need to use community software for that.
The Desire is available unlocked and network unbranded - that's how I bought mine - but that still includes Sense. It does get updates quicker than network branded handsets. And it is possible to update to this software: the unbranded ROM Update Utility (RUU) has been leaked and is available on the net, and you can use this to remove network branding (by loading the unbranded software instead). There are still a couple of things to consider about this:
* The RUU does wipe the phone of all user apps and data (factory reset)
* It will check your phone's ID to decide whether it should run. The idea is that unbranded RUUs should only update unbranded phones, and branded one update phones with that branding. There is a trick to bypass this check, by making your SD card into a "goldcard", which allows you to use the unbranded RUU on a branded phone. This is described in the FAQs linked from the sticky post in the root subforum.
* If you need to return the phone to your network provider for repair they will expect it to have their software on it, so you'd have to use the appropriate branded RUU to put that back first. For most networks a suitable RUU is available, but there are exceptions (Orange UK for example).
When people talk about "debranding" a phone, that's what they mean - using the RUU to install generic software instead of the network stuff.
If you root the phone then need to return it for repair, you need to use an RUU to unroot it first - otherwise your warranty is void.
Personally I sympathise - I did root, and my phone is Sense free and in my opinion better for it. But I'd certainly recommend reading up and making sure you understand all of the implications before doing that.