This is the first I've heard of Tidbit and I have to say that it is absolutely ingenious!!!
If they do open source the code, I strongly hope that webmasters would actually replace obtrusive ads with the mining protocol and not just add it in addition to ad revenue.
It's been posted before and quite frankly, it can very easily be abused as a malicious feature. Especially if the user is unaware of their being complicit in mining.
It should be noted that their intent was to be purely opt-in. Stealing a user's CPU/GPU cycles is extremely bad form. (It's bad enough when ads/tracking code does it by accident.)
CPU bitcoin mining doesn't really do much. The idea is nice, but it would take way too long to be competitive as a form of revenue generation if all it had access to was the CPU.
If they do open source the code, I strongly hope that webmasters would actually replace obtrusive ads with the mining protocol and not just add it in addition to ad revenue.