Here are some tips on panning in AutoCAD.
In some earlier versions (up to AutoCAD 2006) of AutoCAD you could hold down Shift and use the middle button (or wheel) on the mouse lock lock panning vertically or horizontally.
In later versions (AutoCAD 2007 and on) you will notice that orbit (3DORBITTRANSPARENT) starts instead.
Update: A reply in the discussion group also mention that if you first press down the middle button (or wheel) and then Shift it works as before.
What you can do is using Ctrl and the middle button. It works the same way as before but you need to be stable on you hand to make it only pan up or down or to the left or right.
But there still is a way to have it work as before. Use the command PAN (or 'PAN for transparent use) instead in combination with Shift and the left button or the middle button.
This button for Pan Realtime is also available on the Standard toolbar (at least in AutoCAD 2008)
If you want more control on panning you can use the command line variant of PAN namely -PAN in combination with either Ortho on or Polar on. That way you can select to pan exactly a specified distance or even at a specified angle.
For more on PAN enter PAN on the command line in AutoCAD and press F1 for help.
I wrote up more details on this as a response to a question in the AutoCAD discussion groups.
In 2006 I could hold the shift key down, then click, and drag my middle
mouse button and I could only pan the contents of the viewport in a straight
90 degree direction. Now in 2008 when I do this I get some 3D orbit command
and not a pan. Can I change this back?
Update via email from Tracy Lincoln that I was approved to add here.
Nice PAN tip, keep up the good work!
You can use the command alias P for PAN and -P for -PAN... you can also use 'P and '-P for transparent use of these commands.
If you have right-click time sensitivity turned off and shortcut menu selected (Options > User Preferences > Windows Standard Behavior Area > Right-click Customization...) then you can right-click with nothing selected and select PAN or use the a key.