setting variables within ifeq section doesn't work?

[Jason Lunz]

Is this expected behavior?

        $ make --version
        GNU Make 3.81
        Copyright (C) 2006  Free Software Foundation, Inc.
        This is free software; see the source for copying conditions.
        There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
        PARTICULAR PURPOSE.

        This program built for i386-pc-linux-gnu
        $ cat Makefile 
        ifeq ($(A),x)
        A:=y
        $(info A set to $(A))
        endif

        .PHONY: default
        default:
                @echo A is $(A)
        $ make 
        A is
        $ make A=a
        A is a
        $ make A=x
        A set to x
        A is x

I would expect A to be set to "y" whenever its previous value is "x",
but that doesn't happen.

Jason