I know this is an old thread, but I thought I'd mention I've posted a "Calibre Library" application to the Android marketplace. It connects to your Calibre server and lets you browse and download files wirelessly. If your server is visible on the Internet, you can even use your 3G connection, Otherwise with wireless it can connect across your own home network to your server without the need to connect a cable.
http://www.humanoidapps.com/calibre_library