This theory got some attention on the internet a few years back, but I haven't found a definitive source on it.
Both Ditto and Mew are genderless pokemon, with similar color schemes for both their normal and shiny sprites (images from Serebii.net):
The ditto normal sprites are somewhat more purple than the Mew sprites, but earlier generation Ditto sprites were a bit pinker, and the shiny sprites are almost exactly the same color.
In addition, both Ditto and Mew weigh 8.8 lbs (4 kg). They both learn Transform naturally at level 0. There are a bunch of places where Ditto can be caught in the original generation I games, but in Yellow the only two places are the Pokemon mansion (where Mewtwo was cloned from Mew) and Cerulean Cave (where Mewtwo lives). Mewtwo is also called the only "successful" clone of Mew, so there's reason to believe that there were failed attempts, which could be Ditto.
Of course, everything above is just fan speculation, but it's gotten a fair bit of attention and I'd imagine someone from the Pokemon Company has commented on it. What's the official word on this theory?