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!