I hate giving estimates. I'm always wrong.
Still, I am becoming more comfortable with the idea of "sometime during 2012". That seems safe enough. (Time will tell.
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I can give a brief progress update at any rate. The first (and one of, if not the biggest) steps toward the iPad version is having Synthesia behave in a way that makes sense on a touch device with no keyboard. That includes redoing basically the entire interface using familiar touch paradigms, making controls large enough that they're comfortable to tap, removing some of the older crufty complexity, finding a better answer to anything that uses the right mouse button, preparing for retina graphics support, etc.
Once that's finished, things are in a much better place to actually start working on a port. Though redoing the entire interface is an absolute mountain of work. The great news is that I've been whittling away at it for months now and a huge portion of it is finished. I daresay we're approaching a development preview in the next few weeks.
During this process I've been very careful to make sure I'm not just shimmying things around and making them a little bigger. Wherever I could, I've added small improvements (e.g., shortcuts are no longer limited to only three bindings and input/output are getting a lot more flexibility). This is also the first time in the project's history (since the first release in 2006!) where all the screens are being updated at once. Things are going to feel more internally consistent now instead of each screen almost feeling like a different app. The technology updates required for the UI revamp are also really exciting: inadvertently and with almost no extra effort I've been able to include resizing support and the early stages of support for localization to other languages. I am guessing window resizing will be coming to 0.8.5 and multiple languages just after that.
Long story short: the first mountain of work is proceeding very smoothly, there are tons of side-benefits for the future, the new UI dev preview is coming up soon, and the iPad version should be ready at least by the holidays. (Ideally it will be months sooner... but I can't promise anything because I'm always wrong.
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There might also be at least one other surprise in there too, but I'm not 100% sure so I'm not going to announce it yet.