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.
Short answer is yes.

At a maximum 24x drive speed, you can read 265.92Mbits/s off of a DVD. This assumes that the Xbox has a 24x drive, which I have not been able to validate. On the other hand, Xbox hard drives are SATA drives, which, at their slowest, have a data rate of 1.5Gbits/s

This means that, worst case, the Xbox can read from HD minimum 5.64 times faster than it can from the DVD drive.

While you may be able to get to the data 5 times faster, that doesn't mean it can't be processed faster. Math aside, I have noticed much quicker load times when playing GoD or games which I have installed.