Works here.

OS?

Transfer method? (Cable, Card Reader, Copy to Folder and import?)

Use a card reader.

There are some issues with the 5D MKII and certain apps on Snow Leopard, and apparently now LR4. Why not use Canon's EOS utilty that came on the software CD with your 5D MKII? I also suggest installing Canon Digital Photo Professional, which will allow you to read all picture information including Canon's proprietary data (such as micro focus adjustment settings, etc)...which LR can't read. Even though I do 99% of my image processing in LR, I find them very useful from time to time. The latest versions that are supported on Snow Leopard are:

Available here:

http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/ eos_digital_slr_cameras/eos_5d_mark_ii#DriversAndSoftware

You will need to install them first from the Canon CD that came with your 5D MKII, and then download and install the above updates.

You can report this as a bug here:

http://feedback.photoshop.com/photoshop_family

You can also download and try LR4.1 RC here:

http://labs.adobe.com/technologies/lightroom4-1/

It will install as separate application and will not overwite your current LR4 installation or affect LR4 on uninstalling LR 4.1 RC.

IMHO -  I never use LR's 'Card' or Camera' import tools. There have been numerous issues posted in the LR forums of users "losing files" during LR import direct from camera or memory card. I'm sure most of these are due to "user error" or "equipment malfunctions," but that's my point "It Happens!"

I use Windows Explorer or the Canon EOS Utility to transfer image files direct from the camera or card reader to a target hard drive folder. Only after all files have been transferred to the hard drive folder do I import them into LR. If you have mutiple memory cards you can import and start review of the first card's files in LR while the others are downloading. It doesn't take much longer using this two-step procedure, but lost images can mean having to reshoot a job, or worse, lost forever. Before deleting or formatting a memory card, I also check the files imported into LR for editing ability and in Windows Explorer for file count match of the hard drive folder to memory card .

Knocking on wood, I havent lost a single camera image file since LR 1.0.

All previous release candidate (RC) versions had the ability to be installed as a "standalone application," at least on Windows OS platforms. I uninstalled LR4 before loading LR4.1RC, so was not aware of the behavior you experienced. Not sure why they made this change, but LR4.1RC is what you want to be using anyhow until the 4.1 final release is available.