Well, the phone manufacturers have to port Android to their phones, so updates won't be available until the manufacturers apply their porting mods to the update and then push it out. That is the down side of not buying a Nexus One (G1). That is also the reason I was disappointed when I read Google is shutting down their phone hardware sales.

Unfortunately, manufacturers tend to lose interest in supporting their hardware after the initial sale is made. The classic example is all the printers that worked fine under XP, but the manufacturers never bothered to create Vista/Win7 drivers, so they became paperweights).

The Google reference phones have the advantage for updates since, by definition, Google has already done the porting for those phones. In theory, Open Source has an advantage since the community can do updates. That theory is tempered by the reality that the community needs to include people that are capable of doing the porting and enough hardware information needs to be available for people other than the manufacturer do do porting.

It is not (directly) dependent on the manufacturer. It's dependent on how many barriers your manufacturer and/or carrier put between the original Android source, and your phone.

In the case of HTC phones with Sense, they have to add a lot of engineering work between "Android version is available" and "that version has Sense added". They need to make sure all the hardware drivers still work, update them for updated kernels, etc.

If the phone is a "Google Experience Device" then it's very close to the original source, and should get updates sooner than the rest.