See GoF: During the Pensive-Memory of Karkaroff's trial K names Snape as Death Eater.

There is no time given, when Snape changed or changed not sides. It must have been between Trelawney's Prophecy and October 31. I thought it might have been the moment Snape realised LV would hunt the Potters. Reasons might have been some feelings for Lily or the Wizard's Debt (which wasn't repaid as the Potters died later).

But then - Dumbledore might have lied to clear Snape.

 

It seems I need to clairify my position. Snape had most likely turned already as Indicated (Though Dumbledor lying about it isn't out of the question.) but How much would you really trust someone who came to you with nothing but his word that he's repented and some nebulous information that may or may not be accurate? By Giving harry relativally Unharmed to Dumbledor, Snape gave tangible proof of his 'Conversion'. Certainly it (The attack on the potters) would have been realized eventually, but the child could have 'Died of Exposure' or 'Been Executed By a fellow deatheather' at that point, and snape could have claimed Ignorance, and Dumbledor would certainly have weighed this strongly in the greasy bastards favor.

 

The statement JKR gives on her website concerning Snape and October 31:

Not entirely helpful her statement is.

 

There's still the possibility that Snape was there on October 31, just not under the invisibility cloak. Maybe he was there with Voldemort, or watching from a "safe" distance, and was the one to inform Dumbledore that Voldemort had fallen? (I'm summarising without researching here, so feel free to correct me)

 

That's what I suspect, too. JKR deliberately didn't write he wasn't at Godric's Hollow. Even the Rumour Snape was hiding under ... is chosen / written in a way to avoid the question / answer if Snape was there at all.

JKR might simply try to keep the speculations going or Snape was indeed there. We might learn more about it later this year.