Answer by ajk for single keypair for all regions

If you can import your own RSA public key, then you can share an Amazon generated key across regions:

1. Using the key you want to share, log in to an instance in the region where it was generated.
2. Open ~/.ssh/authorized_keys
3. Find the line with the key you want to share (the end of the line should show the keypair name you specified when you generated it)
4. Copy all of the line up to the first character of the keypair name. Should be 382 characters.
5. Save the copied data to a file locally (e.g. to the machine you launched the SSH session FROM).
6. Log on to the EC2 console and switch to the region that you want to share the key with.
7. Select the "Key Pairs" management
8. Click "Import Key Pair" and point it at the file created at step 5. You probably want to give the keypair the same name as you gave it in the region you're importing it form.
9. Launch instances using this keypair. You should be able to log into instances in the new region using the same private key as in the region where you generated it.