Below is the list of PCI DSS rules, and if a company were to use a PaaS provider like Meteor/Galaxy, which rules each party would need to be responsible for. I was going to ask whether Galaxy might support PCI compliance, but after reading through the docs, I'm guessing that's pretty unlikely. While it's pretty straightforward on the application side, there is a whole lot to do on the hosting side. In any case, here's my research in case anyone else finds it helpful.
Galaxy
1. Install and maintain a firewall configuration to protect cardholder data
2. Do not use vendor-supplied defaults for system passwords and other
security parameters
5. Protect all systems against malware and regularly update anti-virus software or programs
6. Develop and maintain secure systems and applications (security patches)
7. Restrict access to cardholder data by business need to know
8. Identify and authenticate access to system components
9: Restrict physical access to cardholder data
10: Track and monitor all access to network resources and cardholder data (Lots of logging, including database access, some of it which needs to be viewed by the company daily)
11: Regularly test security systems and processes. (WAP monitoring, network vuln scanning and pen tests, IDS, change detection)
12: Maintain a policy that addresses information security for all personnel. (Includes "contractors and consultants who are “resident” on the entity’s site or otherwise have access to the cardholder
data environment.")
Company
3. Protect stored cardholder data (Encrypt data, either disk-level (Galaxy), or manually in code, with db hooks and complicatedly-managed keys)
4. Encrypt transmission of cardholder data across open, public networks (SSL between client + server and between server + processor)
6. Develop and maintain secure systems and applications (General web app/programming security, some taken care of by the framework, some not)
7. Restrict access to cardholder data by business need to know
8. Identify and authenticate access to system components (This has a whole lot of things, like rate limiting login attempts, lockout time, 15min idle logout, inactive account removal, but it seems you can get around implementing this in your app by just not letting your users view their own CC data.)
11: Regularly test security systems and processes. (App-level pen tests)
12. Maintain a policy that addresses information security for all personnel.