If I understand correctly, there is no string expression input based way to do this, in 10.1.
SingleOutputMapAlgebra is still supported, I assume for backwards compatibility. i.e.>>> import arcpy
>>> print arcpy.__path__
['c:\\Program Files (x86)\\ArcGIS\\Desktop10.1\\arcpy\\arcpy']
>>> print arcpy.gp.SingleOutputMapAlgebra
<function <lambda> at 0x1748C3B0>
>>> import arcgisscripting
>>> gp=arcgisscripting.create()
>>> gp=arcgisscripting.create(10.1)
>>> print gp.singleoutputmapalgebra
<Tool: 'Single Output Map Algebra'>
However... I find the new raster object map algebra syntax to be much easier to use and much more flexible. Also, it's more future proof than relying on something that may not be available later on.