At its recent Google I/O conference the search giant ran a session entitled High Performance Audio on Android. One of the apps that this featured was Raph Levien's Music Synthesizer, an emulation of Yamaha's DX7 synth that's just been updated.
As you'll see in the video above, the latency level does appear to be good enough for real-time playing, though we're guessing that performance will vary from device to device.
The app is free from the Google Play store, though, so you've got nothing to lose by downloading and trying it on your Android phone or tablet.