I needed to move two Amazon EC2 instances from one AWS account to the other. Background story is mostly what these guys wanted to do:

http://austinmills.name/?p=26

and

http://entxtech.blogspot.com/2008/03/how-to-move-amazon-ami-ec2-image-from.html

However, instead of sharing the image like these guys did. I just logged in on the machines of the source account and bundled them with the private key, certificate and account number of the destination account.

Then I uploaded the machine to an S3 bucket owned by the destination account and registered the AMI in the destination account. Booted and instance and it worked like a charm. Another way apparently is to bundle the image, upload it and then share it with the destination account.

But I think that was not necessary in this case, since I only had to move the server from one account to the other.

So just in case anyone has a similar situation, there's nothing stopping you from bundling a runnin instance with cert, pk and account number of the destination account.