Here's another possible workaround, cobbled together from the bits of info I gleaned out of this thread, and my own experimentation.
Download & install the free AutoHotkey utility from the .com website of the same name.
Open Windows notepad and create the following text file: I've attached a copy for your convenience as a .txt file (you'll need to rename as described below), but just in case it doesn't post intact, here's what it should contain:
#IfWinActive, AutoCAD 2011
-::
Send {Alt}{Alt}-
return
Save the text file, giving it the filename of altdash.ahk. Be sure it has the .ahk file extension, not .txt. Rename the file as needed.
In the Windows file explorer, right click on altdash.ahk and select run script. This should be an option if you have installed AutoHotkey.
Now every time you hit a dash in AutoCAD, it'll be as if you typed the following keys <ALT> <ALT> <-> as individual keys. Entering Alt two times forces AutoCAD to flip the menus shortcut keys (the underlined characters that show up in a menu when you press alt once) on and off before issuing the desired dash/minus sign. You may notice the menu bar letters quickly flash on and off each time you enter a dash at the command line.
If you find this works for you, I suggest right clicking on your altdash.ahk file in Windows file explorer and selecting compile. Then place the resulting altdash.exe file in your Windows startup folder to get it to run every time you start up your computer. For example, in Win7, this folder is likely:
C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup
I've not seen the need for this script in R2012 as yet, but it's helpful in R2011 SP3.
I'm not a big fan of hot key scripting like this, but I don't really see a good way around it here. Turning off the menubar is not really a viable option for some of us. At least the script is relatively harmless. No extra cancels or escapes needed in this workaround, just the alt shortcut menu key cycling. You can always halt the script via the Windows Task Manager. The script name will show up there under the Processes tab as a process you can end if you like.
To test, simply switch from AutoCAD to another program and back. That switch needn't be via <Alt>+ <Tab> either; it might be just be picking off another program off the Windows taskbar. At the command line, enter a dash command like: -LAYER, or start a LINE command that begins with a negative coordinates you enter at the command line. Without the script in effect, the dash will pretty consistently trigger a menu popup. With the script in effect, the menu popup should get suppressed and the dash processed at the command line as expected.
