Keep in mind, there are certain things that should not be put on a dimmer. Most fluorescent or arc-type light bulbs will blow if they're dimmed--and they could take the light fixture and/or the switch with them. They have to be specially designed for dimming. This also goes for three-way lamps; don't put a non-dimmable fluorescent bulb into a lamp with a three-way switch. At best, you'll have a burned-out light; at worst, you could have a fire on your hands.
Also, one-speed motors should never be put on a dimmer, or they'll burn out. Most sewing machines are variable-speed motors, so that's fine. The foot pedal is just a dimmer of sorts, after all. I do like the idea of putting a manual dimmer on the machine--it's especially useful for those with limited mobility or sensation in their feet.