Can I run a site on AWS Elastic Beanstalk for PHP?
Yes, we are currently running site build on Drupal 7 on AWS Elastic Beanstalk for PHP.

I am strongly recommend to install and configure memcache for best performance. 
PressFlow is not required replacement for standard Drupal 7 core (but for Drupal 6 is).

Also sites/default/files folder must be mounted from S3 bucket (read here how to manage it https://sites.google.com/site/truthkos/home/tutorials/amazon-web-services/working-with-ec2-instances/setup-a-s3-connection-to-ec2-instance-via-fuse)

For anonymous user Varnish will dramatically increase site performance.