You calculate EVI for IKONOS imagery by using the same standard equation:

EVI = 2.5 * ( NIR - RED ) / ( NIR + 6.0 * RED - 7.5 * BLUE+ 1.0 )

There is also EVI2 (without the blue band):

EVI2 = 2.5 * ( NIR - RED) / ( NIR + 2.4 * RED + 1.0 )

Reference: Huete A.R., Liu H.Q., Batchily K., van Leeuwen W. (1997). A comparison of vegetation indices global set of TM images for EOS-MODIS. Remote Sensing of Environment, 59:440-451

Input the equation in the "Raster Calculator" in ArcGIS.  In the Arctoolbox: Spatial Analyst Tools --> Map Algebra --> Raster Calculator.

Finally, so many papers have been written about EVI.  It won't be hard for you to find, for instance, the range of EVI values for healthy vegetation.

Thank you Eric

Once you have calculated the index following Eric's instruction and have your raster file ready to be analysed you can use the tool "zonal statitstics as table" to extract values by polygons in a table (Spatial Analyst - zonal statistics - zonal statistics as table). This tool will allow you to get several statistical measures using polygons to sample your raster file (i.e., mean, median. std dev, min, max, range, etc). A nice check would be running the extration for distinct land covers and compare them. Make sure the output values are in the correct range of the indices you are working with as well as the values you are getting make optical sense in terms of the different targets you are considering in your analysis.

Thank you Ralph

I think care should be taken when generating EVI from any satellite other than that provided by the MODIS. The reason is simple, most of the calibration parameters for the conversion of EVI from NDVI consider the Measurement system of the MODIS sensor.

Thank you