What you'd look into is DRM libraries to protect games. Basically, any DRM library should work and I think I remember some people on the forums mentioning that they've been successfully using that.
Here's one forum thread:
How to protect Unity games from copying/abuse
Another relevant forum thread:
Copy protection or Copy protection for Windows game?
One solution that is frequently used for games:
SoftwarePassport, Unity itself uses PACE - which, from how I ready the Website also includes all the infrastructure for your serial numbers etc.
And here's a forum thread of someone struggling with integrating DRM (might also help when looking into that):
struggling to hook up a 3rd-party DRM
A little inspiration on how to "creatively deal" with piracy:
Arkham Asylum: Creative DRM
If you're developing for iPhone, you're lucky because this is handled for you by Apple. The bad news: Apps from the AppStore to do get cracked, too. But that's a general problem - there's no such thing as total security, even with DRM. So, eventually, you might look into Fighting piracy by sueing.
Of course, you could also "roll your own" system: You'd set up a database, some Web service for the software to check back, create a way to generate your own serial numbers, a Web interface for your customers to register / authorize their installation.
One could also think of a combination: 3rd party copy protection for your game + your own system for authorization / serials etc.
Finally, if you're going through a portal there's a chance that they have a solution that you'll have to use. So, in general it's usually a good idea to not worry about copy protection too early in the process.