LIke many home audio systems, my Altec Lansing computer speaker system has a switch to add a stereo-like spaciousness to mono sound. It sounds really good. I don't mean reverb, but rather what seems to be a frequency crossover formulation.

Does anyone know what's being done to the signal to produce this effect?

I don't have it with me so I can't quote the procedure but I seem to recall Jay Rose's Audio Postproduction book has a section on how to create quasi-stereo from a mono signal.

There are many methods. If it's in circuitry, it would probably be some complementary comb filter, usually implemented with all-pass filters.
The idea is that if you present each ear with a different set of frequency bands, you will perceive stereo. Decorrelation is the process, although I would call this local decorrelation. So, imagine the spectrum divided into bands. Odd bands go to the left ear while even bands go to the right ear.
Low freuencies are not spread in these circuits, as we are generally unable to localize them. If you want to test if this is the method, run a swept sine (sine wave gliding up in frequency) through your speakers. You should be able to hear the sine wave flip-flop between the speakers.

If you have a 31-band stereo equalizer, try boosting the left channel odd bands and right channel even bands. Then, cutting the left channel even bands and right channel odd bands.