Long post warning
The story Snape tells Bellatrix when she accuses him of treachery seems to be the same story he told Voldemort, a bit shorter perhaps, but essentially, it's his explanation for his dubious behaviour.
Now, at first sight, Snape's story appears to be thoroughly plausible, but if you look at it more closely, the holes are evident.
1)Snape's explanation of why he took up his position at Hogwarts and the story Dumbledore tells Harry fit together very well, leaving the impression that Snape indeed "spun Dumbledore a tale of deepest remorse", namely that sending Voldemort after the Potters was the greatest regret of his life. However, if you include some 'outsider information' and a bit of logic, the whole story seems rather like cover-up tale.
We're supposed to believe that Snape overheard the first half of the prophecy, the half that oh-so-conveniently does NOT include the part that warns Voldemort of the child's power, and then was discovered by the bartender. But why does Trelawney, who fell in Trance and therefore doesn't remember her prophecy, remember Snape's interruption, if he was indeed thrown out before the prophecy ended? The memory Dumbledore showed Harry in the pensieve didn't include any signs of Snape either. So Snape must still have been there after the prophecy was over. Now Dumbledore, who, as we know, has to see the best in people, immediately deduced that Snape would carry that information to Voldemort, otherwise he wouldn't have sent the Potters into hiding. Nevertheless, Dumbledore did not arrest Snape or erase his memory, no, he let him run off to his master. Voldemort of course was delightened. Then, in his overlordly wisdom, he decided he could do with yet another spy, and of all his numerous Death Eaters, he chose Snape. Snape, who had just personally exposed himself as a Death Eater to Dumbledore, and not somebody unknown to the Order. And since Lupin said that, in the first war, the Order was outnumbered twenty to one by the Death Eaters, Voldemort had plenty to choose from. Anyway, Snape's sent to Dumbledore, explains how deeply he regrets having passed on that information, and Dumbledore, without further proof, welcoms him with open arms.
Why is it I don't believe that story?
If you put together all those facts, especially Trelawney's memory and the fact that Snape still only passed the conveniant first part of the prophecy, it seems to me as though 1) Snape indeed changed sides (as most people believe anyway), and 2) he changed side before the prophecy was made. That also fits better with the fact that Dumbledore states Snape changed sides *long* before Voldemort's fall. I now wonder if Dumbledore possibly wanted Voldemort to hear that explicit part of the prophecy, after all he'd never have disappeared that night in Godric's Hollow and marked Harry as his equal if he hadn't known. And Dumbledore seemed very reluctant to tell Harry the details of that particular incident, didn't he?
2)When asked about Dumbledore, Snape says this: "The duel with the Dark Lord last month shook him. He has since sustained a serious injury because his reactions are slower than they once were."
This is very obviously a lie. We know from Dumbledore that it was Snape who helped him get rid of the ring, so Snape knew it was the ring that caused the injury. It had nothing to do with reactions. Can you imagine Dumbledore saying to Snape something along the lines of "My reactions have become much slower lately, so I had to put this very nasty ring on my finger. Honestly, it chased me! Damn thing." Of course...
3)And then, of course, there's the incident at the Ministry of Magic, or more precisely, the fact that Snape alerted the Order because of Harry's cryptic warning. It would have been so easy for him to say he hadn't understood a word.
To me, it seems as though Snape didn't know of Voldemort's plan to steal the prophecy. That makes most sense, even if Voldemort trusts him, considering Snape spent dangerously much time with Dumbledore and was supposed to teach Harry Occlumency, the risk of telling him would have been too high. I expect he contacted Voldemort after he contacted the Order and then recieved his orders to remain behind. Anyway, the fact that he had orders from Voldemort regarding that incident means that Voldemort knows Snape knew about the mission. Consequently, it is easy for Voldemort to deduce who alerted the Order of the Phoenix. And that is something that Snape, in my opinion, cannot explain.
So. Snape is lying to Voldemort, and the closer you look the more obvious the lies are. And I for one don't buy Voldemort unconditional trust in Snape, I didn't believe for one second that Wormtail was supposed to "assist" Snape. Can't it be that Voldemort did not trust Snape, but thought he was too valuable to be killed because Dumbledore trusted him? With Dumbledore dead, Snape has fulfilled that purpose, and Voldemort might change his attitude. Or do you believe the Dark Lord trusts so easily?