I have heard nothing, but I am hoping for one as well. He seems to be one of the few AH writers who appreciates the butterfly effect and the general workings of chaos theory. As well as being a half way decent writer too. I remember reading in a foreword that he was spending a lot of time on the 1632 universe basically writing 3 series based on it or around it. I don't know if 1812/24 where a diversion or another attempt to spawn some fanfiction that he could then publish and make some money on... So who knows?
Just looked on Eric Flint's "Forthcoming" page http://www.ericflint.net/index.php/forthcoming/ (last updated in December) and there's no sign of any work-in-progress related to 1812/24. The last I see of any discussion of this series was a May 2007 post on his main page http://www.ericflint.net/ stating that Baen was taking over the series from Del Ray and that he had signed a contract for two additional novels in the series.