For all we know, Regula 1 became the Genesis Planet. After all, the Genesis device was programmed to turn a dead planet into a living one, and Regula 1 was the only dead'ish planet anywhere near as far as we know. It sounds quite unlikely that the Genesis device could have converted something like the Mutara Nebula into the eventual planet, against its original programming, as the expert Marcuses seemed to agree that the device had no flexibility of programming ("couldn't cram another byte into it") and Khan had no reason to tamper with the programming anyway.
If warp drive was an absolute necessity for escaping the effects of the Genesis detonation, then Regula 1 would obviously have been well within the range of those effects, as the detonation happened aboard a ship that had traveled from Regula 1 to the detonation location at impulse...
Timo Saloniemi
This is one of those horrible plotholes that gets to me. The general consensus is that the Mutara nebula was converted into the Genesis planet, however there is no logical basis for this. Sure, we saw the Mutara nebula essentially dissipate with the detonation of the Genesis device, but it just doesn't add up. I'm inclined to go with Timo on this, unless of course another lifeless space object in the system was terraformed by the Genesis wave?
If Nicholas Meyer ever decides to do a Robert Wise esque director's cut to TWOK, I think they should clearly show what actually became the basis for the Genesis planet. Cutting the sequence of the Enterprise warping away from the system (lets face it, the Enterprise going to warp was enough to ensure us that our heroes escaped the detonation) and then replacing it with a newer CGI sequence of Genesis wave eventually impacting with a planet or planetoid would be a cool addition.
As for reasons as to why the Mutara nebula cannot be the Genesis planet....
1) Its a gaseous space body with very little mineral content, not enough to form the raw materials of a planet.
2) Its size is inconsistent with the Genesis planet, if the nebula was indeed the Genesis planet, then why is it not as big as the nebula?
3) The Genesis planet is in orbit with a star, some might argue that the star was part of the Genesis matrix, but this is inconsistent with Trek in general...if they could make artificial stars, why would they ever need to bother with energy? Just create a small artificial star within a warp engine and you'd have infinite warp drive, no dilithium needed.