U-He Satin is probably the only digital version that exists. I discovered this capability yesterday while researching the issue—I'd been aware of its existence as a tape emulation plugin before, but hadn't had a reason to look more closely since I already have the Ampex ATR-102 and Studer A800 from Universal Audio, which are extremely accurate reproductions of popular studio tape machines from the 70's. (The Ampex was a 2-track mastering deck, while the Studer was a multichannel machine, very similar to the one used to record the score for Star Wars.)
It looks like the U-He plugin is a more general 'tape effect' rather than an exact recreation of all the electronic nuances of one particular machine, like the UAD plugins. But it does appear to be the only software solution for decoding a track that has been recorded with Dolby noise reduction. Dolby themselves have never made a digital version, so either their hardware or this plugin would have to be used to get the right sound from the film prints.
Assuming the Wikipedia article on Dolby A is correct, it shouldn't be terribly difficult to recreate the exact multiband processing necessary for decoding; it's simply a matter of setting the thresholds and attack/release times for the compression and expansion to the right values. The U-He plugin is well viewed for its quality, so in all likelihood its reproduction of the Dolby process is an accurate one; and given its high rate of internal oversampling, any aliasing distortion should be virtually nonexistent. It would, however, be necessary to ensure that the analog signal from the film print has been digitized at the same reference level that the plugin operates at (ie, 0 VU = -18 dBFS, or however the plugin is set; looks like it has a variable calibration capability). Otherwise the thresholds of the processor will be incorrect, and frequencies will be boosted or lowered at the wrong levels, yielding an inaccurate sound. The input to the plugin can be trimmed to the appropriate level easily enough; but the importance of using proper calibration cannot be overemphasized.
Satin doesn't have Dolby SR capability, so it wouldn't be any use for film prints encoded with that, but I'm not sure how useful that would be anyway. SR is much more complicated than Dolby A, and hardly ever used for music recording since digital was taking over around the same time, so I expect it wasn't worthwhile for U-He to try to implement it.
Dolby B is not compatible with Dolby A. And if I remember right, Satin has some issues with how it decodes.
What issues? I haven't seen anything about this . . .