PCI DSS applies to all entities ... that store, process or transmit cardholder data.
(PCI DSS v2 Introduction)
If you process credit card details, you're subject to PCI.
If you transmit credit card details, you're subject to PCI.
SSL / network encryption is one of the requirements for meeting PCI DSS (Requirement 4). There are others; you'll need to assess what you're doing in order to determine which you need to pay attention to. You should look at the Self Assessment Questionnaires to determine which scope likely applies to you.
If the server is outside your scope and you're only concerned with the app on your iDevice, you're probably writing a Payment Application, and you need to worry about PA-DSS compliance instead. That's more serious, there are no self-assessments for Payment Applications, and no organization can use an un-approved Payment Application without being in violation of the PCI DSS.