Photographers with more than one camera body, or who use several cards in a session, would benefit greatly from being able to import from more than one card into Lightroom without having to start separate import sessions individually. However the Lightroom Import dialog doesn't appear to allow for importing from more than one Source at the same time. Is it possible to import more than one card at a time (assuming you have more than one card reader)?
Yes, this can be done. If you ignore the 'Devices' part of the Lightroom import dialog and instead look at the 'Files' part, ensuring you have 'Include subfolders' checked, you can cmd+click (Mac) or ctrl+click (Win) to select more than one source to import from.
There is a video showing how this is achieved at this page on Petapixel. This works in Lightroom 3 and 4.
In Jared Platt's workshop, The Ultimate Lightroom Workflow, he advises against 1) importing from the camera and 2) using the same catalog for all your projects.
He recommends that you need to manually copy the images off the memory card and organize them into folders before importing into lightroom. However, it's a whole workflow concept so you really need to watch the videos to see why it works so well and what's wrong with this current approach.
If you're on a mac, I've found it faster to copy each card to disk via a Finder copy, and then import everything from disk in one import session. If you use a multi-card reader, you can do multiple copies in parallel to a folder, and then import that folder once everything is on disk.