Tampa_Gamer had something on the go - but I'm not sure how far along he is. Brad over at the se5.com forums is also working on one as well.
I will probably give some attention to this area in a short time for the Balance Mod as well. I'm looking to improve the layout of the colonies and add additional buildings and stuff.
GGmod has a fairly advanced ground combat system in the works...
Including fighters with "smart bombs". AKA slow missiles which get to drop from the sky level.
Infantry and militia depend on whether you choose "Pacifists" and/or "Quadripedal" racial traits. Ground Vehicles (AKA tanks) and Ground Bases/Fortifications.