It seems suspicious that Snape told Voldemort when Harry Potter would be moved from his home. At that point of time, he had Voldemort's complete trust, having killed Dumbledore, why did he need to perform an extra attempt to help Voldemort find Harry Potter?
From the Deathly Hallows, page 688:
"You will have to give Voldemort the correct date of Harry's departure from his aunt and uncle's," said Dumbledore. "Not to do so will raise suspicion, when Voldemort believes you so well informed."
Snape does not have Voldemort's unconditional trust. In the books, many people (especially Bellatrix) are shown to resent him and distrust him, pointing to his apparent loyalty to Dumbledore.
The information about Harry leaving from his house is leaked by Snape, but he makes sure that Voldemort does not know about the Polyjuice potion. It's just a part of walking the fine line between winning the enemy's trust and helping his own side.
If Snape hadn't told him, despite the fact that Snape MUST have known, Voldemort would have had to assume one of two things:
a) Snape was no longer trusted by the Order - vital information was withheld from him. His usefulness as a spy is ended - if he's untrusted, the Order won't tell him anything important, and may use him for disinformation.
b) Snape cannot be trusted to give Voldemort accurate information. His usefulness as a spy is ended, as Voldemort has no one else in the Order who can serve as independent verification for information Snape gives, and Snape is too good at Occlumancy for his mind to be read.
In either case, Snape's usefulness to big V (and thus, to the Order as a double-agent) ends, likely with his death.
On the other hand, look at the results: moving a vulnerable target during a prepared ambush, the Order suffers very minor losses. Yes, some important characters suffer, but overall the losses are much less than could be expected - the ambush was supposed to leave no survivors. The Order acted heroically and skillfully, and gave as good as they got...because Snape told them the ambush was coming. And, of course, lived to serve them more.
Edit: As Katie's answer shows, this was planned before Dumbledore's death to ensure that Big V trusted Snape - doing so allowed him to continue working against him from within the Death Eaters despite having been cast out of the Order.
I just wanted to add to the first comment here, with regard to Dumbledore telling Snape to tell Voldemort. Yes, in order to gain the unconditional trust of Voldemort.
Dumbledore's reasoning was that he wanted Snape to become Headmaster, and protect the school from the Carrows.
Or just accept the obvious answer: It's a plot hole. Voldemort already knew that Snape was not trusted by the Order from the point where he killed Dumbledore at the end of the previous school year. Even if the plan to move Harry had been agreed upon before that, there was no way for Voldemort to have known that. There is, therefore, no logical reason why Snape could not have said that he didn't know.