Perhaps you have and oversized piece of art that you have to scan in two pieces. Or maybe your scanner isn't large enough for your originals.
Either way, you're faced with scanning your image in two pieces and pairing them up digitally.
To be sure, if you're very careful with your initial scans, you can do this manually with a little effort.
However, with Photoshop CS3, you can let the app do the work (and, in some cases, do it better than you could unaided).