I don't think there's a confirmed source, but there's certainly indications, for example, an AWS post that they use ECC memory for the GPUs in their cluster GPU instance

http://aws.typepad.com/aws/2010/11/new-ec2-instance-type-the...

Given that they list both the standard memory and GPU memory next to each other, but only put ECC next to the GPU memory, it seems relatively likely to me that the standard server memory is not ECC.

But Nvidia only sell Tesla's with ECC. I don't really read that implication into this.

On the other hand, the fact that https://forums.aws.amazon.com/message.jspa?messageID=203167 has never been answered suggests they do not, as they would answer affirmatively if it was true surely. Of course they may use a mixture.

EDIT: Interesting that James Hamilton is on their team and thinks they should use it http://perspectives.mvdirona.com/2009/10/07/YouReallyDONeedE...

I can well imagine that newer machines have it, and older ones don't, and slowly over time they'll end up with everyone using ECC.

The costs involved were a lot higher 3-5 years ago than they are today.

I was not able to find Amazon declaring that they don't, but neither do they say anywhere that they do. For example they describe the hardware specs here: http://aws.amazon.com/ec2/instance-types/ If they did have ECC RAM it is unlikely that they would keep it secret, especially given that ECC RAM can be twice as expensive.