Thanks to Bulbapedia, I've just noticed an interesting excerpt from Platinum:
I'm guessing that this is said by the temple guardian when all three Regis are brought in, but she definitely didn't have that much to say in Diamond and Pearl. It is possible that even in Platinum the game checks to see if the Regis were caught in Sinnoh, with the Hoenn Regis yielding a different response. I'd check this myself, but I currently don't have access to a DS or the game script.Once after a volcano violently erupted, the Snowpoint Forest, rich in natural resources and beauty, was transformed into a sea of flowing lava.
Then, out of a bright blue light, Regigigas appeared. Along with Regirock, Regice, and Registeel.
Together, the four of them halted the volcano's eruption, thus saving Snowpoint City from destruction.
Later Regigigas transformed itself into a stone of life and fell into a deep sleep, while in order to protect Regigigas, the legend says that Regice, Regirock, and Registeel transformed into three guardian pillars made of the basic elements of rock, ice, and steel.
Regardless of the specifics, this legend comes as a surprise to me. I was very disappointed in the way the Regis were incorporated into Platinum, since I had expected that the Braille messages from Generation III would be followed up on. Even though I'm glad that at least some story is told about this trio, I'm puzzled by the lack of ties to the Hoenn story. I used to think that what we saw in the Platinum event was Regigigas creating the Sinnoh Regis (as implied by the Pokédex entries), or as I preferred to see it, summoning the ones from Hoenn (stranger things have been known to happen). But now it is clear that the Regis have been in Sinnoh for a very long time.
So there are two sets of Regis, and it would appear that the people of Snowpoint viewed them rather differently from their Sootopolis counterparts. Let's review the braille messages from Hoenn:
While there is a common element of showing gratitude to the Regis, the people of Hoenn decided to seal them because they had reason to fear them, whereas in Sinnoh the Regis decided their own fate and did so to protect Regigigas. What is the reason for this difference?In this cave we have lived.
We owe all to the Pokémon.
But, we sealed the Pokémon away.
We feared it.
Those with courage, those with hope.
Open a door. An eternal Pokémon waits.
It would be nice to know that Platinum does, in fact, distinguish between the two sets of Regis, but chances are that Game Freak didn't put that much thought into the script. I have to say that it is quite disappointing that none of these legends sheds light on Regigigas' Slow Start ability. I used to think that it was cursed by the same people who sealed the Regis away, but that is obviously not the case.
Right now my theory is that the volcano mentioned in the legend is none other than Stark Mountain; Regigigas saved Snowpoint by towing that part of Sinnoh away. I don't see any other obvious way in which it could have stopped the eruption from affecting Snowpoint, although the legend does say that the four Regis actually halted the eruption. It would be a good idea to check the Japanese version of the text.