I'm building an app that needs to accept credit card payments with something like active merchant. In terms of security, is it possible for it to live on heroku and use authorize.net (or similar) as the payment gateway?
What if it needs to store the credit card numbers?
Edit
Would not be forwarding users to authorize.net.
The simple answer is yes, I believe so, but beyond that it depends.
You can set environment variables for various keys and other values related to the third party service(http://docs.heroku.com/config-vars), or just check them in and deploy them.
If you are using the hosted payment service for authorize.net, and forward to their site, you don't need ssl yourself. If you will be hosting the form where credit card number and personal info are submitted, then forwarding this to authorize.net via their API on the server, you need to set-up ssl for heroku (http://docs.heroku.com/ssl) so that your form is secure.
Now, it is one thing to accept payment via credit cards and just pass it thourgh, it is another to save credit card numbers and other private information. Without pointing you to various security standards docs (i.e. PCI DSS applies here), I will simply say that unless you absolutely have to, don't store CC numbers and related personal information, just forward to the gateway and make sure you aren't logging those fields (http://guides.rubyonrails.org/security.html#logging). If you do need to store credit card data, I think you need to have more control of the database and server to reach compliance, and I don't know a general cloud host like AWS or heroku that you can use and do this (maybe some other SO user will correct me). Using a payment gateway like authorize.net, however, can get you there.
I'll also point out that different states now have laws about storing sensitive data (like MA, where I live), so yet another reason to avoid doing it unles it is essential to your business model.
For a somewhat dated, but good general discussion of PCI compliance, look here: http://broadcast.oreilly.com/2009/02/pci-in-the-cloud.html