Hi,
Some math symbols seem to be failing with the latest nightly build. 
Minimal example:
\starttext
\section{blah}
AAAAA $a \xRightarrow b$ BBBBB.
\stoptext

I see this in the output:

ConTeXt ver: 2012.07.04 23:48 MKIV fmt: 2012.7.6 int: english/english

system > tex > error on line 3 in file demo.context: Undefined control 
sequence ...
...
...

...
\math_arrows_construct_single ...s \p_mathspacing
\endcsname \p_mathspacing ...
\math_arrows_handle_finalize ...{#7}{#1}{#4}{}{#8}
}\fi \else \ifsecondargume...
<to be read again>
b
l.3 AAAAA $a \xRightarrow b
$ BBBBB.
! Undefined control sequence.

system > tex > error on line 3 in file demo.context: Undefined control 
sequence ...