Bt-i is only effective for larvae of certain members of the fly family, like fungus gnats. Don't waste your money if that's not what you have.

Why not try horticultural diatomaceous earth?

I have some diatomaceous earth, but my experience is that it gets washed up by watering. Also, using the stuff indoors doesn't sound pleasant, from a respiratory standpoint.

Bt-i is only effective for larvae of certain members of the fly family, like fungus gnats.

Hence, the fact that it didn't work helped convince me that my fungus gnat hypothesis was wrong and to take a closer look at the critters ;)

I pyrethrined last night. I also managed to smash one of the adults to get a closer look at it. Pale, almost translucent yellowish body, translucent white wings, total size about 1-2mm. Didn't look like a whitefly, though, and the soil isn't whitefly habitat anyway.

Have you thought about using nematodes?

http://gardeningzone.com/index.php?cPath=25_75

that is where I ordered Steinerema feltiae... I'll let you know in a few days if my assorted fly problem has gotten better =P

You wrote "...the fact that it didn't work helped convince me that my fungus gnat hypothesis was wrong..." (and that after 1 1/2 weeks)

As it turns out, one must use Bti with every watering for at least 3 weeks to disrupt the gnat's life cycle.

I considered nematodes, but they're just so expensive. Plus, once they finished up the insects that were infesting my soil, they'd starve, as this is an indoor setup, not outdoor.