Okay, just a couple of small corrections to previous posts. DesertJets is right, the construction and tree clearing you saw north of Terminal D was most likely the new runway (which will be 8L/26R as opposed to 9L/27R). However, depending on your vantage point, which you said was looking past gate D-12, it could be construction for the new cargo area. It is supposed to be located between the threshholds of the current Runway 26L and the future 26R. The new cargo area and its respective cargo buildings are supposed to have the capacity to handle 20 widebodies simultaneously, so it is no small project. Like I said, that depends on whether you were looking north or west.
As for Terminal A, like Drerx7 said, the south side has been completed for about a year and a half, and the north side is presently under construction. It is in the final construction phase, with some jetways already in place, and it should be done sometime in the next few months.
The area that will be Terminal E is undergoing ground clearing. It will be south of the present Terminal D, and will open initially for domestic operations in 2003. However, until the new FIS (Federal Inspection Services) building is completed in 2004 it will solely be used for CO domestic ops. Terminal E will have two piers and about 20 gates, and will be solely used by CO and COEX (and perhaps their alliance partners, I'm not sure) for international services starting in 2004.