Apparently they are working at a fix that should be available in short time. Of course it's always a risk to build a business based on someone else's proprietary technology, but there is definitely a demand and I doubt they invested much: the potential is well worth the effort.
I don't think Apple has enough to win a lawsuit:
- Beeper is basically implementing the iMessage protocol through reverse-engineering, which means their implementation is not infringing Apple's copyright.
- Beeper is interacting only with Apple's public servers. They have not gained access to private or internal network infrastructure.
- I doubt there are patents at play protecting the iMessage protocol. That might be a possibility though.
Reverse-engineering is nothing new and it's been established as legal under many circumstances. From my understanding, Beeper is doing nothing that is "legally risky". Said that, Apple doesn't need to be right to sue...
The more interesting aspect is that it's now clear that an interoperable iMessage is not only technically achievable, but likely pretty easy to accomplish. This means Apple's motivation to avoid interoperability with iMessage is mainly strategical as opposed to technical.