I thought I'd give the mailing list a small change of pace---instead of asking how to do it, I will show you :)

The short answer is:

\starttext
My Concrete Poem

\startlines
All       the white        space
    will               print           !
\stoplines
\stoptext


However, \startlines..\stoplines is not available when defining a macro. I've documented the solution to this situation on my blog:
http://drippingdigital.com/blog/2010/09/typesetting-poetry-in-context/

Thanks to Wolfgang Schuster for helping me work it out!