Hi Ed,

> I have projected text onto a curved surface and extruded the
> text, but was unable to cap the ends because they are not planar.

I think that Burr and Colin answered this above, but also you may want to check out this previous post for some illustrations on what to do in that case:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3024.4

The method there is to trim the surface with the curves, that will then basically create the piece that you want to have as a cap right there.

Then when you extrude, extrude the cap surface itself, rather than just the curves.

When you extrude a surface, the surface's edges will create side walls the same as when you extrude curves, but it will be able to place a cap at the other end even if the surface is not planar because it can place a copy of it there.

- Michael