Wow... I'd almost forgotten about this thread before someone worked their necromantic arts upon it. Time to make the post I'd thought about when I first read it, but then had eaten by forum errors when I tried to post it.
Something that people should keep in mind is that the barrage of nuclear death fired at an incoming starship by modern day Earth isn't as feasible as people seem to be thinking.
The only delivery system capable of hitting a space-based target would be an ICBM. In theory, these could achieve escape velocity and blast the RT's starship. In practice, that's not what they're designed for. They're meant to be fired at targets on the Earth's surface using a suborbital flight path. Also, going by the movement rates the game gives, even the slowest freighter could probably dodge aside from the missiles. Being intended to target stationary locations, ICBMs won't really have much in the way of course correction, so just moving out of the way will dodge them.
Essentially, shooting down a starship with ICBMs would be like shooting down a helicopter with an anti-tank missile. In theory, you could do it. A few people have in fact tried it and got it to work. But you really wouldn't want to rely on it working as your main plan for survival.
Now, modern technology probably could build something much more suited to target space-based enemies if there was a reason to do so. But there isn't such a reason. If the Rogue Trader didn't immediately conquer/destroy Earth, then ten years after the world'd governments realised they needed them, the barrages of hundreds of nukes ready to fly at any invaders would be sitting in silos all over the planet. But that assumes that they're given time to build these.
Also, the idea of the modern world being willing to commit suicide by nuking the planet and denying an invader it as well it probably not realistic. "Give me Liberty or give me Death" is a nice slogan to fuel patriotic fantasies, but actually taking it to the logical extreme when liberty isn't an option any more is beyond most people. You'll decide that your own friends and family will all die as well, just to spite your enemy? That's something most people won't realistically do.
And consider this- think about the people who currently run the world, the ones who'd have the authority to order the nuclear suicide- and the interests that control them. Really think about them. Then ask yourself- if a Rogue Trader told them he'd be willing to ensure they and their descendents had a position of power and wealth in the new regime if they co-operated in the takeover of the planet... Would you trust them not to sell out you and the rest of the population to live in the sort of hell the average Imperial citizen endures?
The Imperium of Man has ten thousand years of experience in taking control of human-occupied planets, and outright war is only one of the tools in it's inventory, even if it is the favourite and most often used one.
(And the forums went into some sort of maintenance state as I was in the process of typing this out- it's like someone doesn't want me to post this here...)