If you haven't watched it, or don't care, stop reading. If you have, here's my take:
The bomb went off in 1977, and that changed things. The biggest thing is that the bomb and the release of energy from it sank the island. On a smaller level, since there was no island, and no button to push, Desmond is on the flight that won't crash because he didn't push the button.
Image via Wikipedia
(If there was no Island, then why would Jacob 'save' Locke when he was pushed out the window? Why would he interact with Sawyer and Kate?)
A bomb exploding the island brings up a paradoxical riddle akin to the grandfather thing. If I go back in time and kill my grandfather, could I really have killed my grandfather since without him, I wouldn't have been born?
This is worked out on the theory that time isn't linear but has multiple waves. It's like the multiverse theory, whereby there are a million alternate universes and any action one could take is taken in an alternate universe. Think Sliders. (But this is getting too geeky already).
And therein lies the greatness of Lost. It's not just a TV show, it's 60 minutes of messing with our minds.
In Lost, since they blew up a bomb in 1977, it must have destroyed that time line. So the plane lands, drama ensues (and the drama will be like the Oceanic Six Drama, happening in real time)
The strange thing is the alternate reality that takes place in the island, an alternate time line in what might be an alternate universe where 'things' can take any shape and magic is real. See Sayid, coming back to life.
(In the 'real' world, the magic might be Jack helping Locke walk again.)
So, to recap: the bomb did two things. First, it blew one timeline to the present, and skipped 30 years, meaning the "present" can be totally different from the non-nuclear-bomb present that the passengers crashed into.
Second, it created an alternate non-Jacob interfering timeline wherein the Losties are just people who took a flight and landed. They all have some serious shit, so this timeline will explore their serious shitness -- though Hurley's "I'm the luckiest person in the world" leads one to believe this new non-Jacob timeline is also much different.
Regardless, both times are impacted by the bomb. It will be fun to see how.