I assume you already know how to SSH to Amazon using an Identity File:
ssh -i ~ec2-keys/id_your-keypair email@example.com
You can use a similar command to connect to your EC2 instance via SFTP:
sftp -o IdentityFile=eec2-keys/id_your-keypair firstname.lastname@example.org
TextWrangler allows you to open a file from an SFTP server, but it doesn't allow you to configure an Identity File as you did when SSHing or SFTPing from the command line. To work around this limitation, you simply have to configure your SSH client so that it knows the Identity File you want to use. Create ~/.ssh/config if it doesn't already exist and add a line specifying your Identity File location:
echo 'IdentityFile ~ec2-keys/id_your-keypair' >> ~/.ssh/config
Now you're ready to use TextWrangler to connect to your server and browse files. Choose: File -> Open from FTP/SFTP Server... Then enter your server publisher DNS name, check the SFTP box and enter ubuntu as the User name. You can leave the Password: field blank.
After you hit connect, you TextWrangler will allow you to browse the file system of your EC2 instance, choose a file, and edit it. When you save the file, it will update directly on the remote EC2 instance. That's all there is to it!