Question: Microsoft sells "full" (non-Arcade) games on its Xbox Live Marketplace as digital downloads ("Games on Demand"). On average, are GoD loading times significantly faster than that of the disc version "installed" to the hard drive? Or are the access times identical because the data and retrieval techniques are identical?

Answer: 
Depends on the game.

If the game has an installer (ala Halo Reach), it will load significantly faster via disc than via the GoD version.

But for the most part, the Games on Demand version will run a lot faster during loads (but some of the loads are specifically hard coded times, such as Mass Effect elevators, so that won't ever get faster).

If you're trying to decide between as disc and GoD, you'll get a better price on a disc, but GoD will probably load faster due to the constant read speeds off a single drive instead of transferring data to RAM, reading from the disc and the HD.
I know someone who has an Xbox 360 Slim and using a lot GoD. I personally use more HDD and DVDs on a fat Xbox 360 and I can assess that the difference between him launching GoD and me launching from HDD is not significantly faster. We are in a university residence and the internet connection is somewhat playing yo-yo, sometimes I can be faster than him, sometimes not. 
And I'm using an old version of the Xbox.

To answer directly the question, the answer should be **no**, it's not loading times faster. But it might be a little faster .

I think the two main criterion are:

* Version of the Xbox
* Speed of internet connection