\starttext
\startformula \startalign
 v &= u + at \\
 h &= ut + \frac12 gt^2 \\
\stopalign \stopformula
\stoptext


---->>

ConTeXt  ver: 2012.07.17 16:42 MKIV  fmt: 2012.7.17  int: english/english
(...)
! Missing $ inserted.

system          > tex > error on line 3 in file test.tex: Missing $ inserted ...

1     \starttext
2     \startformula \startalign
3 >>  v &= u + at \\
4     h &= ut + \frac12 gt^2 \\
5     \stopalign \stopformula
6     \stoptext

Thanks - 

Matthias