Is there any service that lets me as a developer implement purchasing via SMS for the user (for Android)? I've found something called Zong, yet it seems that they haven't released a SDK yet? I believe Google's In app billing system only supports credit cards?
What are good billing services to go with, that are easy to implement, are user friendly and have a sufficing agreement when it comes to the money?
Is there a particular reason you want to bill via sms? Perhaps you can find a solution that does the next best thing, operator billing?
There are a bunch of providers for mobile payments, I put together this list of various mobile monetisation companies with some brief information https://docs.google.com/spreadsheet/ccc?key=0AtLMbkIIPH0VdEs4M1Z3anNmU1JOY2xXejN1MVdMaXc#gid=0
I know that Bango offers operator billing for sure, take a look at some of the other providers in the "In App Billing" category of the doc I linked to.
It also depends a lot on what geographical coverage that you need/want. There is varying coverage over providers so think about that too (even Android in app billing has limited coverage).
Also, some network operators offer APIs for in app payment via operator billing.
Telefonica/O2 have their Bluevia APIs for payments and some other stuff, the only drawback is this can only be used with Telefonica/O2 customers in certain regions across Europe and South America.