The letter was prompted, at least in part, by the death of Mohsen Ruholamini, whose father is an adviser to Mohsen Rezai. Ruholamini was arrested in Tehran on July 9, and his father spent several days trying to tracking him down; he eventually was sent to the morgue where he found his son's body, brutally beaten.
According to an article in this morning's NYT, protests continue to be only sporadic, with a large security presence preventing any large gatherings. However, the article also says that the clerical establishment has still not completely signed on to Ahmadinejad's victory: only one of the marja-e-taqlid (those who are deemed a "source of emulation") has congratulated Ahmadinejad, while three of the nine have condemned the government's crackdown.
The article is not clear as to whether the elder Ruholamani is more on the liberal or conservative end of the spectrum (I've read about Rezai before, but I can't remember his political positioning), but it sounds as though the impact of the security crackdown on a prominent/establishment family has re-invigorated some of the criticism of the Iranian government/Khamenei.