Yeah, they do check how the sites look on mobile devices. They might also test them under the average 3g data speed, though there are a lot of developers who just use the office wifi and assume it'll work fine everywhere else.
I generally only make "enterprisey" web apps these days, but I will make sure they are usable all the way down to a 2G connection.
And you don't just do this on the $800 top of the line phone, you try it on other OS's, last year's model, a model from 3 years ago, that 2 year old phone from my mother in law that is still crapped up with games and other stuff so it runs extra slow, and then you adjust accordingly.
And the thing is this really doesn't take that much longer, and in the grand scheme of things budgeting for a few older devices to test on is cheap as shit.
