Now Sidney van den Bergh has written The Curious Case of Lemaitre's Equation No. 24, with some new clues:
The 1927 discovery of the expansion of the Universe by Lemaitre was published in French in a low-impact journal. In the 1931 high-impact English translation of this article a critical equation was changed by omitting reference to what is now known as the Hubble constant. That the section of the text of this paper dealing with the expansion of the Universe was also deleted from that English translation suggests a deliberate omission by the unknown translator. ...Okay, the translator was dishonest, but that does not explain the rest of the physics community. Lemaitre was well-known, and his theory was denounced by Einstein and others before Hubble. Einstein said, "Your math is correct, but your physics is abominable." Someone should have had the decency to admit that Lemaitre was right, and well ahead of Hubble.
(Lemaître 1927) to have been the first to find both observational and theoretical evidence for the expansion of the Universe. His observational discovery was based on the published distances and radial velocities of 42 galaxies. Lemaître’s theretical result was based on the finding that the Universe is unstable, so that pertubations tend to grow. These results, which were published in French and in a relatively obscure journal, anticipated the work of Edwin Hubble (1929) by two years. It might therefore have been appropriate to assign the credit for the discovery of the expansion of the Universe to Lemaître, rather than to Hubble (Peebles 1984).
In summary it appears that the translator of Lemaître’s 1927 article deliberately deleted those parts of the paper that dealt with the determination of what is presently referred to as the Hubble parameter. The reason for this remains a mystery.
Hubble supposedly had better data, as he used the world's biggest telescope. But his data was off by a factor of 10 or more, and not nearly as convincing as he pretended it was.
Jean-Pierre Luminet also has a new article saying:
The year 1931 can undoubtedly be called Georges Lemaitre's annus mirabilis. Indeed, major contributions to relativistic cosmology by the Belgian physicist and priest appeared within a few months: ...Luminet says that Lemaitre removed the mention of God, but we do not know exactly why. Maybe he knew that the editors would not accept it.
Lemaitre initially intended to conclude his letter to Nature by “I think that every one who believes in a supreme being supporting every being and every acting, believes also that God is essentially hidden and may be glad to see how present physics provides a veil hiding the creation".
Update: David L. Block has a new article arguing that Lemaitre was censored by Hubble and others in A Hubble Eclipse: Lemaitre and Censorship.
Update: The new paper
Did Edwin Hubble plagiarize? responds to Block, and disputes the idea that Hubble got his ideas from Lemaitre. That may be, but it still seems clear that Lemaitre did the work first, and that there was some sort of conspiracy not to credit him.