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.
(based on what I have read before and my own experience)
The load time are pretty much the same, as the content of the games including updates are taking up equal space, having the HDD loading the same files. I am not sure how the loading time from a none updated game loads when comparing disc installation and GoD installation. But I doubt there will be any difference at all.
If you look at various fora, you will see that its an issue discussed numerous times.
All with the same conclusions, that the difference isnt noticable.

Bonus info:
A comparison between loading from a disc and loading from the HDD, shows an improvement on about 15% faster load time from the HDD.

* 
* <http://support.xbox.com/da-DK/games/manage-game-library>

(the last link is in Danish, but change the languege to your prefered)

\*\*NOTE I have not bought the same game twice to do the experiment myself.\*