You might not be able to mount the Canon on the desktop unless it has a setting in the camera menu to switch it to USB Mass Storage mode. Many camera brands can do this, but many Canon models cannot.
It's still pretty easy to get images off it, though. In addition to iPhoto, you can use Apple Image Capture to select and download images to any folder, or any other software that supports the XSi like Canon EOS Utility (included) or Adobe Bridge/Photoshop/Camera Raw. If the XSi doesn't turn up in those apps, make sure you've got the latest versions.
I don't have the XSi, I have an older model, but
according to reviews, the XSi still doesn't have USB Mass Storage support. Without that support I don't think it's going to show up on a Mac desktop.
You may be seeing it show up for drag-and-drop on a Windows Vista desktop, but the link above says that drag-and-drop in Windows only supports JPEG files, not RAW files. Are you getting just JPEG files on the Windows desktop? If so, and if you intend to use the superior RAW format, then basically you don't have drag-and-drop on Mac or Windows, if the review is correct.
