A few things to watch if you are using the free tier is the number of I/O's you use on your EBS (you get 2 million/month free) and your bandwidth (you get 15 GB/month free).  I've not tried it but if you plan to run it continuously then you might want to keep an eye on these things.  I'd be interested in hearing how it goes.

Also, I understand if you want to stick with CentOS, but I wanted to mention it would probably be super easy if you wanted to just use one of their Ubuntu server images.  You could just install the O/S then type 'sudo apt-get install bitcoind' and probably be done.