[Windows Phone 8 Q&A, Help & Troubleshooting] GoodDayToDie:
For the record, wired tethering does work on the Samsung Ativ S. The trick was finding the drivers for my 64-bit PC.

First, you need the Diagnosis app for Samsung phones. This is included, but hidden, on each phone. To access it, type ##634# (##MFG#) into the phone dialer. After it's run once, you can launch it from the Apps list like normal.

Next, to access the USB control, type *#7284# in the numpad that the app displays.

Select "QC RmNetComposite" from the USB Path Control radio button options. This will take the phone out of its default MTP (Media Transfer Protocol) mode, and make it act as a Qualcomm USB modem. The phone will need to reboot when you select this option; let it do so.

When the phone boots up again, it will not appear in Windows Explorer or the Windows Phone app. You'll get a "Found new hardware" notification, but the odds are that the drivers will fail to install automatically. We'll need to install them manually.

To do that, first download the attached ZIP file included with this post. Extract its contents somewhere handy.

Next, open Device Manager. This can be done from the Start menu/screen, or by opening Computer Management, or by typing "devmgr.msc" into Start search or the Run dialog and then hitting Enter. It will require Administrator permissions.

You should see four "Qualcomm composite device" entries that don't have drivers. We only need two, maybe even one, of them. It's possible to tell them apart, but for simplicity's sake I'm just going to tell you to do the next steps for each of them.

Double-click one of those entries, or right-click on it, and select Update Driver.

Choose the "Browse" option, and navigate to the folder you extracted from the ZIP file. You don't have to go any deeper, so long as the "Include Subfolders" option is checked.

Hit Next and, if a warning pops up about unsigned drivers, choose to install anyhow. There is actually a signature; the drivers will work on 64-bit Windows. I don't know what that complaint is about. If it just says that it couldn't find a driver, that's fine; that's one of the two devices we don't need, probably.

Once you have the Qualcomm USB Modem and (possibly) Qualcomm Diagnostics devices installed, you're ready to configure the modem in Windows. To do this, open Network and Sharing Center (or Network Connections in XP) and chose the Create new connection option.

Select a "Dial-up connection". Don't worry; it's neither as slow nor as noisy as a classic modem.

This next part is probably carrier-specific. For T-Mobile US, it was ridiculously easy: type a name for the connection (doesn't matter what), in the place for the number you dial, put "*99#" (without the quotes), and leave the password and username blank.

Try to connect! If it doesn't work, look up the WP7 settings for wired tethering to your carrier; they should be on this forum somewhere.

When you want to get back to normal (MTP) mode on the USB port, just repeat the first three steps but choose "MTP Mode" instead of "QC RmNetComposite" and let the phone reboot again.

Something which may help: I found that the PC would lose data connection while the phone used it. To make the PC's data connection more reliable, turn off data usage on the phone. The modem should still function. There are a ton of other tweaks to configure the modem driver which might also help, but for basic usage, the steps provided here work.

I recommend copying the driver bundle onto your phone (or its SD card) so that if you need to tether a PC that isn't already set up, you can get the drivers without needing Internet access. It's quite possible to copy Zip files to and from the phone via MTP mode.