It's inevitable. It's as simple as that.
Now, would they have the nerve to stream full blown video commercials during loadings of full priced games? Surely not, but you can sure expect a lot of demos, trailers, small/cheap downloadable games to come with a a healthy amount of video commercials.
The broadcasting networks are suffering every years from the loss of potential advertisment viewers (multiplicity of channels and thus people hoping on other channels during ads, PVRs, competition from other media, such as internet or videogames, etc..). This hurts the networks, but also the ad centrals which handle the commercial placements.
So, if people are not watching TV ads, but instead are playing videogames, well then the nice folks in charge of advertisement groups will simply bring the advertisement to them.
And people won't be able to do much against it. Just think about it, the console manufacturers, with their platform OSes, and the publishers, with their software, have access to a huge and extremely targeted sample of folks, who are forced to watch their screen (once the loading is done, the game can start right away.)
It's just way too obvious not to happen. It's a matter of when, not if.
And the fact that you can target a particular crowd is making this a fantastic ad stream for the marketeers. You can display toys ads during Spongebob loadings, expensive sneaker ads during Madden loadings, etc...
And as Acert pointed out, you will surely have to face longer than what they should have been loadings. We might see every, well thoses that rely a lot on ad streams, games install themselves on the HDD at the first start, some sort of initial long loading. Just to give them a reason to have a long ad before every play.
Anyway, that's the price to pay for... Hmm... I don't know what we get in return, as consumers, but at least we know the price we have to pay for it, heh.