Hallo Hans,

I have played with positioning text in layers. I noticed that parameter "location=t" 
  \setlayer[MyLayer][...,location=t,...]{...}
behaves the same way as "location=c". It seems to me it is the bug in optimized definition of 
\alignedbox. If I use commented version of \alignedbox from source file core-box.tex
it is OK. Can you look at it, please?

Test file:
------------------------------------------------------------------
\definelayer[PositionText][position=no,corner=buttom,height=\textheight,option=test]
\def\PosText(#1,#2)[#3]#4{%
  \setlayer[PositionText][x=#1,y=#2,location=#3]{#4}%
  \ignorespaces}
\setupbackgrounds[text][background={PositionText}]

\switchtobodyfont[20pt]
\starttext

\PosText(.3\textwidth,.3\textheight)[c]{Center}
\PosText(.3\textwidth,.3\textheight)[t]{Top}
\PosText(.3\textwidth,.3\textheight)[b]{Bottom}
\PosText(.3\textwidth,.5\textheight)[l]{Left}
\PosText(.3\textwidth,.5\textheight)[r]{Right}
\PosText(.7\textwidth,.5\textheight)[tl]{TopLeft}
\PosText(.7\textwidth,.5\textheight)[tr]{TopRight}
\PosText(.7\textwidth,.5\textheight)[bl]{BottomLeft}
\PosText(.7\textwidth,.5\textheight)[br]{BottomRight}

Ap\hskip1.5em
\alignedbox[b]\hbox{ApB}\hskip1.5em
\alignedbox[c]\hbox{ApC}\hskip1.5em
\alignedbox[t]\hbox{ApT}

\stoptext