I have my own suspicions about the jump, and I am inclined to believe most explanations are too inelegant for Sherlock to actually perform (and I really do use elegance as my measure of least-most probable). Thus, any explanation involving body doubles or jumping on a truck or bus, I have a hard time believing.
My theory is that Sherlock JUST DID JUMP. He did, however, have a means of breaking the fall presumably by the exact same means that helped one of the CIA operatives survive in 'A Scandal in Belgravia'. If you recall, Sherlock was asked on the phone by Lestrade:
LESTRADE: And exactly how many times did he fall out the window?
SHERLOCK: It’s all a bit of a blur, Detective Inspector. I lost count.
And from Mrs. Hudson when we hear Neilson fall from the room:
MRS HUDSON: Ooh. That was right on my bins.
Since we know that the CIA operative (Neilson) was able to survive multiple falls and he was still in standing-condition, I propose that Sherlock did the exact same thing: foreseeing that he was going to have to jump even from a height much higher than what Neilson had to do, he did so but was more severely injured. The fact that Sherlock also was sitting (rather than standing with a cane) in the final reveal supports this idea.
For me, it's certainly plausible that Sherlock gave the biker some of that hallucinogen to get Watson to believe Sherlock was dead. However, one thing I don't necessarily understand is why he would have required Watson to be alone in that 'poisoning'...in that if he also got the pedestrians nearby affected, they would further corroborate the idea he fell and died (which is what their suspicions were, likely, anyway). Of course, it does make sense that he kept Watson so far away (because then he might have identified the means Sherlock had to break his fall).
Molly assuredly was part of the plan (his means to get past the coroners and proclaimed dead) and because of how Irene got proclaimed dead as well, I have every reason to believe Mycroft was in on it, too (explaining his disinterest when he reads about it).
Lastly, I also believe that this would have had to have been convincing EVEN IF Moriarty did not shoot himself, which makes 'jumping onto a truck' less viable...not to mention it seemed clear to me his downward falling trajectory couldn't possibly have landed on the street, anyway.
On a TANGENT, one of the more popular theories I've read was that it was a double and that Hallucinogen + Facemask is how he faked his jump (akin to how the kidnapped children were afraid of Sherlock). But I refute the facemask idea for reasons of inelegance, as well...not to mention that it seems fairly clear how those children were scared. Richard Brook was a real persona created by Moriarty (as 'the Reader') in which he used the television show to tell stories of Hansel and Gretel and (though not shown on the series), with a very good likeness of Sherlock over television, condition the two orphans (and probably lots more children!) to be afraid of him when they finally saw him.
That's my theory, anyway.