The internal keyboard harmonizers will not do that, nor will the majority of the external harmonizers. They "read" the notes, or chords, that you are playing from a pre-designated section on the keyboard, and select the harmony notes from them. Changing the note that you are singing does indeed change the structure of the resulting vocal "chord", but it remains in the key of the chord or notes you are playing on the keyboard.
There are basically 2 modes that the harmonizers use: Chordal and Vocoder. In Chordal Mode the harmonizer reads the chord, samples your voice, and reassigns the samples to the notes selected by the preset selected from your Harmonizer. In Vocoder mode, the harmonizer simply reproduces the exact notes you are playing on the keyboard.
There is and external non-midi unit made by Digitech that works simply from the voice and presets, but I have no personal experience with it.
I hope this explanation helps some. It is over-simplistic. The current harmonizers are quite easy to use and are nearly completly automatic. Having said this, the Korg Pa80 that I just got is the most limited I've seen when it comes to presets, editing, etc. I await a software upgrade. Are you listening Korg? They should take a PSR9000 and copy the harmonizer function.
DonM

[This message has been edited by DonM (edited 06-24-2001).]