Not tested this, but this widget could do what you want:
DESCRIPTION: To Allow the user to
select any shape file in zip format
(that is example.shp – for geometry
and example.dbf - for Attributes, are
only files to be zipped) and display
over the current Map as graphics.
Actually it creates a custom geometry
to take advantage of the coordinate
array format in the shapefile reducing
the impedance and taking advantage of
the compression. This an array of
x,y,x,y,etc... That’s all; it can then
be rendered with any symbol.
ASSUMPTIONS: Only Polygon shape file
to be added is allowed as current
limitation but for sure libraries are
ready that are open to be extended and
customized as you want.
I also remember reading Mansour's blog (Heads up Esri Flex team) and he has a post on how to achieve this in a native Flex app.