Judging from the number of requests that I have had for this particular combination of EC2 features, I'm expecting this to be a very popular post.
The micro instances provide a small amount of consistent CPU power, along with the ability to increase it in short burst when additional cycles are available. They are a good match for lower throughput applications and web sites that require additional compute cycles from time to time.
With this release, you now have everything that you need to create and experiment with your very own Virtual Private Cloud at no cost. This is pretty cool and I'm sure you'll make good use of it.