CPU intensive for who? The client is basically just sending input that might only a be a few bytes. It depends on how twitchy your game is, but since you mentioned mobile I'll assume it's not a 400 apm RTS game. Then for getting the state I'm imagining a fixed update rate that's fairly low like 200 ms at the most? (5 updates per second). Assuming you're not updating hundreds of units at once and are doing sane delta updates I don't think you'd hit anything that would be that CPU intensive to deserialize coming from the server.
The only thing I could find was an article here http://sirisian.com/.../binary-packet/ but it is regarding packing bit by bit, which I am not going to do since I think it would be too CPU intensive for a mobile game.
That said I wrote these two writers. C++ version. C# version. They are simple binary writers/readers which I wrote for someone that was developing between languages. Basically what's going to help you more than anything is sane delta compression. Not sending anything the client already knows about. Your server should be 100% aware of what the clients know and intelligent in building the per client packets. (I'd give you more information, but I removed my old articles online relating to this so I could rewrite them. Been meaning to get back to that).
// edit also I see my robots.txt isn't stopping people from finding my blog. Interesting.