In Android 3 the connection to PC as Mass Storage Controller feature was removed.
Is there a way to access MTP-mounted phone as a drive letter? I want standard file manager programs (e.g. Total Commander) to work.
In theory, there is not a way to access your phone's memory card with a drive letter, because, as you said, Android now connects it as an MTP device, and not as a Mass Storage device. But, there are two solutions:
There are applications like FTPUse and NetDrive that will map that FTP address to a drive letter. I used NetDrive on Windows XP, but couldn't get it to work on Windows 7 64-bits, so now I use FTPUse, and it's working fine.
So, it's not exactly the same, and it's a bit slower, but it works, and I am able to browse folders, erase files, etc, and even use applications like WinDirStat to analyze folder sizes etc.
My method is based on the answer suggesting WebDAV. It could be replicated by an FTP, Samba or other networked file server. Essentially you start a WebDAV server on your android device, and mount it as a network drive. Your phone's IP must be visible to the computer you want to browse the files on. Generally this means you must be on the same local network. This solution exposes your phone's filesystem to your local network, and could cause some security issues. I do not recommend it for untrusted networks.
First get an install a WebDAV server app on your android device. I used WebDAV Server because the interface is simple, it was the first app in my search and the price is right (free!). Start your WebDAV server app and get its address. This should be something like
Next, open Computer in windows explorer. In the tool bar, under the address bar, click "Map Network Drive." Enter the address in the box labeled folder. Click finish. If the mapping fails make sure you can ping the IP address of your phone.
Boom! Your device should now show up in Computer as the drive letter you selected (Z: by default).
You can try to install a WebDAV server to the android device, then mount the WebDAV server as a drive on Windows 7. Here is a free WebDAV server from Google Play (Of course, there are many other WebDAV servers in Google Play). And the steps to mount the WebDAV server as a drive on Windows 7.
http://www.totalcmd.net/plugring/TotalAndDroid.html - this plugin does not give you a drive letter, but beside of this - does the job. You can see all files of your Android device on TC panel and do all regular operations.