This is a simple hack. Copy the following text into a .reg file, and merge it into your registry (I prefer to use .reg files).
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Control Panel\Desktop]
"CoolSwitch"="1"
"CoolSwitchColumns"="12"
"CoolSwitchRows"="4"
This is a per-user change. To change the number of icons displayed horizontally, edit the value for CoolSwitchColumns. To change the number of rows of icons displayed, edit the value of CoolSwitchRows.
I have no idea what happens if you pick numbers that are too big for your screen, but you probably won't like it.
You "have to" reboot for the change to take effect.
Why: According to the Wikipedia article, Alt-Tab is a function of csrss.exe. Csrss.exe is a "Critical System Process," which you can't kill with the task manager. So you really don't "have to" reboot, you just have to reload csrss.exe. Unfortunately I don't know how to do that without a reboot :(