Here's some useful suggestions for getting a MAC address via networking apis: http://stackoverflow.com/questions/677530/how-can-i-programmatically-get...

I've not tried these through Marmalade but they should work. However, the reason Apple deprecated UDID support was to avoid privacy issues with sharing actual device identifiers, plus Apple could also eventually block access to MAC address...

Some background info:

s3eDeviceGetInt/String(S3E_DEVICE_UNIQUE_ID) is the current Marmaldeway to identify a specific actual device. On a lot of platforms this will return the IMEI. iOS doesn't legally support getting IMEI, IMSI (sim identifier). Instead it provides a UDID - unique identifying number generated from hardware IDs (basically can treat like another MAC address). This is what S3E_DEVICE_UNIQUE_ID will return. You can also view this value in iTunes when a device is connected.

However! In iOS UDIDs are deprecated in iOS 5.0 (They still actually worked in 5.0 but will be stopped in later versions). Apple state that apps should use UUIDs. Basically you can call CFUUIDCreate via the EDK to create a totally unique identifier (on iOS devices), but this is not associated with the device hardware or user id and can't automatically be shared between apps (unless you made the user log in via some cloud service).

Marmalade doesn't support MAC address querying via an S3E API because this information has typically not been available on many devices. We might reconsider this, but probably not given the privacy complaints around it.

Check out OpenUDID: https://github.com/ylechelle/OpenUDID
This gets a unique ID in a similar way to CFUUIDCreate but its not controlled by Apple (cant be deprecated) and can be shared between apps.

I decided to force user to register on my web server using his email, store it in device memory and use the email as unique user id.