If you right click in the mtext editor or click on the symbol icon a list of predefined symbols is showed. It is possible to add your own symbols or text snippets to that list.
I can confirm that it still works with AutoCAD 2008. For some reason it didn't work in AutoCAD 2007 and I'm not sure if it works in AutoCAD 2006.
To have this work you need to use the AutoLISP code found at
Below is a cheat sheet showing what the symbols look like. (For a future version of AutoCAD it would be great to also see the symbols instead of the cryptic text codes)
The symbols are Degrees,Plus/Minus,Diameter,Almost Equal,Angle,Boundary Line,Center Line,Delta,Electric Phase,Flow Line,Identity,Initial Length,Monument Line,Not Equal,Ohm,Omega,Property Line,Subscript 2,Squared and Cubed.