Digital Cameras are most sensitive for overexposure; they have no shoulder at all and clip very abruptly. That's why they almost always try to
underexpose the scene; rather than overexpose. This is especially true with newer cameras as people have complained about clipping. So: digital cameras deliberately UNDERexpose the shot, to give that popular blocked-shadow look.
I've used a compact digital camera as a light meter when exposing slides at night. Usually you can expose slide even a stop more; so, when you look at the digital picture, the areas that are blown a little will be alright on the slide. Where digital clips, there starts the short shoulder of slide film. It's not much but it's there. When it looks like this with digital:
Dead Link Removed (look at the blown-out sky), it comes okay with Velvia 50 with the same exposure (plus compensated for reciprocity failure).
But, the ISO values in digital cameras differ very much, more than one stop between manufacturers. In addition, they fake their highest ISO values, or maybe all of them. You really cannot say without comparing a particular digital camera to a particular film. This has come very clear today as I have been wondering these ISO 25600 shots of the new Nikon D3s all around the Web, and they are almost always both underexposed and blocked-shadows, true speed being much lower, something like ISO 3200-6400.
So, the bottom line is that I would expose slide at the same rating than with digital. It can be under or over but you can't know without testing. If I had to place a bet, I would say that it's half a stop under rather than over.
And, about the digital not having the reciprocity failure; it's true but in a sense they have same kind of problem; it's the huge noise buildup regardless of ISO value when using long exposures. Even 60 sec exposure (even at ISO400) can be very fatal to digital image. This is much temperature-dependent and if you have cold winter night at -30 deg. C then you will probably be completely ok with 120 sec exposures.
Now I spoke about d*git*l in APUG
. Forgive me.