Thanks JaneEyre .....I recently did the same thing but cut the spike off a Phal. and cut it into pieces....got 3 keikis out of 8 chunks.....but the reason I was asking ....I keep looking at the cut sprays in the flower shop here, as they are the only orchids we get here that are not Phals.
I would guess it depends on the type of orchid. If I'm not mistaken, the cut flowering spikes need to contain meristematic tissue (phals contain it on their spike nodes). Even if they do, I doubt that it will work as the cut flower industry refrigerates flowers to retain freshness, so who knows how old they are. I have tried to make keiki from orchids in my wedding bouquet with out any luck.