Geoprocessing tools that import data to a geodatabase feature dataset or append to an existing feature class do not perform geographic (datum) transformations on the data.
This is a known issue.
The input data must be transformed to the geographic coordinate system (datum) matching that of the feature dataset or feature class, specifying the correct geographic transformation, before the import to geodatabase or append takes place. This applies whether the output coordinate system is a projected or geographic coordinate system.
The geographic transformation is performed using the Project Wizard (8.x) or Project Tool (9.x and later) in ArcToolbox.
Select the input data, the output coordinate system to match the coordinate system of the feature dataset or feature class that will receive the data, and the correct geographic transformation from the drop-down menu.
Refer to the Related Information section below, and the article "Select the correct geographic (datum) transformation method when projecting between datums". At the bottom of the article, download the list of Geographic Transformation methods and areas of use that applies to the version of ArcGIS Desktop installed on the computer being used.
In Version 10 of ArcGIS, this process transforms data correctly if the following steps are performed:
a) In the Project tool dialog, click the Environments button > Output Coordinates, and change the 'Output coordinate System' from the default to 'As Specified Below'.
b) Select the correct output datum and coordinate system through the dialog.
c) Select the correct geographic transformation(s) to be applied during the conversion process.
d) Click OK on the Environment Settings dialog.