Reprojecting MrSid images with ArcGIS could be a challenge, as you might have to uncompress the file, reproject each band, then re-compress the file. One way to overcome this problem is to use Erdas Imagine. In Erdas, follow these steps:
1. Open the MrSid image in Erdas
2. Select Raster | Geometric Correct | Reprojection.
3. In the dialog that appears, select the new projection and then select the ‘ruler’ on the Geo Correction Tools dialog to calibrate the image.
Another way around this problem is to download and use GeoExpress – http://www.lizardtech.com/products/geo/