If you can set (or even check) the settings manually then you should be able to use the digi-cam as a meter. If you can set the digi-cam to display a histogram, I would suggest using that to judge exposure, rather than looking at the image on the LCD screen.
I do this sometimes when I forget my handheld meter. Obviously its important to consider the differences though, for example with my digital p&s I always set exposure compensation to -1.0 because it doesn't like highlights at all, whereas this wouldn't be such an issue with negative film. As Mike suggests I find it better to use the histogram rather than the LCD image.