Mapping Center Answer:

There is no way to do this out of the box.  I did find the following on the ArcGIS Desktop Discussion Forum - it is a custom button to calculate acreage (note that I have not tried this out myself): http://forums.esri.com/Thread.asp?c=93&f=992&t=271615

You can search for other related resources on the ESRI Support Site.

If you only need to do this once, add a new field in the attribute table and calculate the new acreage value.  Here is the VBA code to use (check the "Advanced" option in the Field Calculator dialog - the first four lines go in the Pre-Logic VBA Script Code area and the last line goes in the bottom one - I assume here that you will call the new field "Acres"):

Dim dblArea as double
Dim pArea as IArea
Set pArea = [shape]
dblArea = pArea.area

Acres=
dblArea*0.000247104369225325 

If you anticipate having to repeat this process a number of times, you could also create a very simple model that will do this.  It would contain the tools for adding a field and then calculating the field value. If you already have the acreage field in the attribute table adn you jsut want to update it, then you can add a first step that deletes the existing field before it adds the new field and calculates the values. 

Alternatively, you could write a Python script to do the same, but in the script it could check for the existence of the existing field before deleting it.

Here's a tip: you can use the model template we recently posted on Mapping Center to start making your model -- this will save you even more time!

For more help with models and scripts, visit the Geoprocessing Resource Center.