You can currently find it in the cubicvr repository under "vis_testbed" and "CubicFX" but those versions are out of date in comparison to the iPhone -- the version to be posted shortly includes many improvements like fractional detection as well as better source comments.
Re:Open source BPM algorithm gone? 3 Months, 2 Weeks ago
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I've verified the link and the svn checkout procedure listed here is working correctly, SourceForge might have just been doing maintenance or something at the time you checked it.
I'm having some trouble getting BeatDetektor to work on the iPhone. I pretty much just copied over the core directory, and some code from main.cpp from the test_app project.