I have the Samsung Windows 8 Developer Preview tablet they handed out at BUILD 2011, and I can confirm that it's going to be very tricky indeed to hit buttons on the desktop.
The Samsung tablet has a screen that's either the same size or slightly larger, and a lower 1366x768 resolution, and I have a lot of trouble using the windows desktop on it with anything other than a mouse (including the digitizer pen -- yes, it's more accurate than a fingertip, but you still have to hold it steady, and holding it steady enough to hit a 4mm target is pretty tough).