I integrated DIRAC2 into an iPhone app so I could modify the playback speed, and it does indeed work in real-time on an iPhone 4. I had to use the lowest possible settings to keep the CPU usage down, but it does play without any skips and I am able to change the playback speed seamlessly.
Running the same project on a 3GS device yielded lesser results - namely that the audio had enough skipping that it wasn't really usable. One caveat to this, though, is that I was running my test on the free version of DIRAC2 which only supports a 44100 sample rate, which is much higher than I need. If the pro version is used and you slash the sample rate down to 22050 or lower, it might work on a 3GS, but don't quote me on that.
Anything older than a 3GS has absolutely no chance of real-time playback.
Hope this helps.