is that the same multiplication that any lens would get from a cropped frame? ... i.e. if i got the sony 11-18mm its lower end would be comparable?
Yes, any DSLR that uses an APS-C sized sensor will have a zoom factor of 1.5 compared to the traditional full frame DSLR and film cameras. That means that your 12 - 18 mm lens will will be equivalent to 18 - 27mm on the APS-C sensors.
You should multiply all focal lengths you see by 1.5 if you want to work out what it actually is on an APS-C sensor.
The sigma website says the lens fits sony as well :s
Sigma makes lenses for most cameras. However, each lens mount (i.e. the connector that links the lens to the camera) is different. So a Sigma lens that has the Nikon AF mount will not work on Canon and Sony cameras and vice versa.