From the Amazon Web Services Blog...
If you have taken a close look at Amazon EC2, you know that the instances are ephemeral. The instances have anywhere from 160 GB to 1.7 TB of attached storage. The storage is there as long as the instance is running, but of course it disappears as soon as the instance is shut down. Applications with a need for persistent storage could store data in Amazon S3 or in Amazon SimpleDB, but they couldn't readily access either one as if it was an actual file system
As you can read in our forum post, we've been working on addressing this.
In the same way that your running EC2 instances, your Elastic IP addresses, your S3 buckets and your SQS queues can be thought of as items contained within the scope of your AWS account, our forthcoming persistent storage feature will give you the ability to create reliable, persistent storage volumes for use with EC2. Once created, these volumes will be part of your account and will have a lifetime independent of any particular EC2 instance.
These volumes can be thought of as raw, unformatted disk drives which can be formatted and then used as desired (or even used as raw storage if you'd like). Volumes can range in size from 1 GB on up to 1 TB; you can create and attach several of them to each EC2 instance. They are designed for low latency, high throughput access from Amazon EC2. Needless to say, you can use these volumes to host a relational database.
Sorry for the big quote but I wanted to get the whole gist of the post in.
The reason this is important for .Net developers is because this, in theory, means that you will be able to access SimpleDb and S3 accounts while running Mono on an EC2 instance. This was supposedly possible before but I never found anyone who got it working. It required some odd work arounds which, pardon the pun, didn't seem to work at all. Because of that .Net developers have been severely limited in what they could do with EC2.
If this resolves that you'll be able to run full fledged .Net applications in an EC2 instance which is pretty exciting.
Admittedly, Mono still has its issues. Most notably is the fact that it runs several versions behind the .Net Framework (it is almost caught up to 2.0 at this point). But being that EC2 is mostly useful for processor intensive tasks and most of those tasks don't require the features introduced in later versions of the framework I think that's close to a non-issue.
Microsoft may very well have an EC2 competitor coming in the next few weeks but until that happens this gives .Net developers something. Beyond that, even when Microsoft gets around to producing something like this I'll feel better knowing it has some competition from Amazon.