To expand on the topic of the SDK: will the SDK be open-source? Will I need to register as a developer or pay a fee to get the SDK? If the SDK is open-source with no registration or fees required, then you have my attention.
It sounds like you've already figured out that the registration would have to be optional, as you're planning to make it open-source (it will be open-source once you release it, it isn't open-source yet :) ).