On Sun, 24 Oct 2010, Vedran Miletić wrote:

> The code
>
> \usemodule[units]
> \starttext
> $y = \underbar{20 \Mega\Bit\Per\Second}$.
> \stoptext
>
> produces space before y and it and y get underbared. MkII produces an
> expected result.

IIRC, \underbar is a text mode command, not a math mode command. You can 
try

$y = \text{\underbar{...}}$

or define

\def\munderbar#1{\text{\underbar{\mathsurround\zeropoint\math{#1}}}}

Aditya