Hi Guy, it might help if you showed an example, there are different kinds of coplanarity like are you trying to take 2 objects that are on parallel planes but different levels and you want to align those? For that there's Transform > Align.

If you want to take some planar curves and relocate them onto the side of a building or something like that, then the Transform > Orient tool that Mike mentions can be good for that, there is some more info on that here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3977.2

Also if you have some curves that are slightly non-planar and you want to make them all planar, using "flat snap" from a side view can work well for that kind of case:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4

- Michael