This program:
\setupbodyfont[lmodern]
\starttext
\type{$\lbrace$} <$\lbrace$>\crlf
\type{$\lbrack$} <$\lbrack$>\crlf
\stoptext

This does typeset a left brace but chokes on \lbrack.
I took my copy of the TeX Book and find \brack as one of the defined delimiters in chapter 17. How come?

ConTeXt  ver: 2010.03.20 22:59 MKIV  fmt: 2010.3.21  int: english/english
fonts           : preloading latin modern fonts
bodyfont        : 12pt rm is loaded
language        : language en is active
systems         : begin file test.tex at line 2
! Undefined control sequence.
<recently read> \lbrack 

                        

l.4 \type{$\lbrack$} <$\lbrack
                              $>\crlf

Hans van der Meer