ATcast,
There is also a program called VDatum
Get VDatum
This is a program that you can enter the coordinates, and it gives the difference between datums, in the US, with the datum being NAVD 88 converting to the WGS84 ellipsoid....
Here is how its used with the FAA...
The different boxes use different methods to calc the GPS difference, Smiths uses a lookup table, while Honeywell calculates the difference.