The focal length of the lens is a physical characteristic of the lens, so your 18mm lens is always an 18mm lens, no matter whether it's attached to a film or a digital camera. The results that you get from it though are equivalent to you using a 27mm lens on a 35mm film camera.
The 1.5x multipier is entirely due to the D50 having a physically smaller sensor than a 35mm film image and has nothing to do with lens design or the fact that it's being used on a digital camera - if your D50 had a differently sized sensor, the multiplier would be different, and if it had the same size sensor as 35mm film, there'd be no multiplier at all. The lens still projects the same image size, but all the information around the edge of the sensor in the D50 is ignored, whereas it is recorded in a film camera. You can think of taking the shot with your digital camera as like looking through a toilet roll tube, whereas with a film camera it's like looking without the tube. If you took pictures with the same lens on a 35mm film camera and your D50 digital camera, the projected image on the film/sensor will be the same size, but only the information in the centre of the shot is recorded by the D50. If you created prints of the same size from these shots, the one from the D50 would appear to be larger or closer because it has cropped all the peripheral information included on the film picture. If you took a photo with a 35mm film camera with a 27mm lens attached, it would be the same as the one you took with your D50 and 18mm lens (ie 1.5x). To get an image equivalent to that taken by an 18mm lens on a 35mm film camera, you'd need to put a 12mm lens on your D50.
Heath