Maybe not the perfect solution, but I've got one for ye. No idea if it works with mice that can scroll on an X-axis, but it works on regular scrollers.

My previous USB mouse came with this nice 'Browser MOUSE' program that allows you to set special mouse button functions. It's the only one I know of that makes it possible to set it so that when you click the scroll wheel, the program switches to 'horizontal scroll mode'. The scroll wheel will work just the same, except it's scroling horizontally. JCS compatible. It appears to break scrolling in MS Office 2007, though, and also in the normal vertical scrolling mode, so personally I'm only running it if I need the horizontal scrolling feature.

You can download it for free from Medion's website, here. This file is a self-extracting ZIP file. Just have it create a folder on your desktop or open it directly with WinZip. You'll find a folder in there named 'mouse'. The setup is in this folder.

Once you've got the program running, double-click its tray icon. At the second tab ('Buttons' or something similar), you can set button 3 (which is the scroll wheel, though you may also choose button 4 or 5 if you have those on your mouse) to the horizontal scrolling mode. Click OK and you're done!