Question

Can I use the same key pair file across all regions? I want to keep the ssh access as simple as possible.

I have set up one file for each region. I tried to upload the same file in different region but it did not not work as expected.

Answer 1

This has meanwhile been enabled by means of importing your own keypair(s), see the introductory post New Amazon EC2 Feature: Bring Your Own Keypair:

You can now import your own RSA keypair (or the public half, to be
precise) for use with your Amazon EC2 instances.

Why would you want to do this? Here are a couple of reasons:

1. Trust – By importing your own keypair you can ensure that you have
   complete control over your keys.
2. Security -You can be confident that
   your private key has never been transmitted over the wire.
3. Management of Multiple Regions – You can use the same public key across multiple
   AWS Regions. [emphasis mine]

This approach is quite beneficial/convenient indeed – I’m using it for a while already, and it has considerably simplified EC2 usage across multiple regions and accounts.

Eric Hammond provides more details and a short tutorial in his article Uploading Personal ssh Keys to Amazon EC2:

Amazon recently launched the ability to upload your own ssh public key
to EC2 so that it can be passed to new instances when they are
launched. Prior to this you always had to use an ssh keypair that was
generated by Amazon.

Accordingly, I recommend to follow his article regarding the (few and simple) steps required to achieve this.