Most people are going to click through any security warning because they just want to get to the site they wanted to go to. If Comcast does this, it would make EVERY SSL site display the warning, making it utterly meaningless.
Alternatively, it's not that outrageous to think that Comcast et al could get certs into the major browsers if they wanted to do so. It's not even implausible to think that at some point, browsers will be legally required to distribute ISP certs to allow for the "safety" of users.
If Comcast makes you install a custom application to keep your certs up, it won't matter if Fx and Chrome block each cert within hours, because Comcast can keep generating and pushing new ones out. And, as above, if the ISP is going to fiddle like this, the actual power held by browsers is greatly diminished -- users aren't going to use a browser that doesn't let them browse without nag screens on every page, even if it is "for their own good".
As I noted in a comment elsewhere in this thread, Comcast (or any other vendor) doesn't need to go through the work of getting certs into major browser. They just need to purchase and use a root signing certificate that works under the existing root CAs that are already in all the browsers.
This is part of why the trust model of the current CA system is fundamentally broken. We need to add a layer that can ensure that we are in fact using the SSL certificate that the site owner wants us to use.
I think you're a bit out of touch as those browser warning pages have changed a lot the last few years. It's actually pretty hard to get through those warnings now in most of the browsers.
Alternatively, it's not that outrageous to think that Comcast et al could get certs into the major browsers if they wanted to do so. It's not even implausible to think that at some point, browsers will be legally required to distribute ISP certs to allow for the "safety" of users.
If Comcast makes you install a custom application to keep your certs up, it won't matter if Fx and Chrome block each cert within hours, because Comcast can keep generating and pushing new ones out. And, as above, if the ISP is going to fiddle like this, the actual power held by browsers is greatly diminished -- users aren't going to use a browser that doesn't let them browse without nag screens on every page, even if it is "for their own good".