Your calculation does not consider reserved instances. Reserved instances allow you to get the cost of running a large instance down to $127/month (not including bandwidth, on a 3 year contract) or $163/month (not including bandwidth, 1 year contract). Does not change the overall picture though.
You also do not differentiate between traffic in and out ("in" is currently free fo EC2 and in any case cheaper than traffic out) - this may be significant depending on the kind of service you run (for most applications it won't be).
Similarly: if you make use of other Amazon Services (in particular S3 which you too seemed to like) this would count as external traffic from servers external to amazon but not from EC2. However, again, for most applications that would probably not change the overall picture.
But i do see one argument that can change the picture: if you use ec2 for overflow, testing and short time workloads and some other hosting (in-house?) for everything else, then this significantly complicates your infrastructure, i.e. you need take care of your in house servers and understand ec2. This may be significant enough to run even long running servers in the cloud.
You also do not differentiate between traffic in and out ("in" is currently free fo EC2 and in any case cheaper than traffic out) - this may be significant depending on the kind of service you run (for most applications it won't be).
Similarly: if you make use of other Amazon Services (in particular S3 which you too seemed to like) this would count as external traffic from servers external to amazon but not from EC2. However, again, for most applications that would probably not change the overall picture.
But i do see one argument that can change the picture: if you use ec2 for overflow, testing and short time workloads and some other hosting (in-house?) for everything else, then this significantly complicates your infrastructure, i.e. you need take care of your in house servers and understand ec2. This may be significant enough to run even long running servers in the cloud.