The problem with it is that it doesn't allow the kind of nested constraints and grouping that, say, Motif does. We could implement this with nested panels, perhaps, and specify constraints on these. However, not all windowing systems support nested windows (e.g. XView), it can mess up tab traversal, and is generally overkill. It wouldn't solve grouping of rows and columns, either, where the window needs to intelligently lay out its children and possibly shrink to fit around them.
So I'm working on 'fake' window classs based on wxSizer, which enables you to have the effect of nested windows, without the need for real nested windows. There will be a wxRowColSizer which can lay out its children, and there is scope for other 'intelligent' sizers. The layout strategy need not be totally top-down as in the present constraint system; the sizer can choose whether to lay out its children first and then consider its own constraints, or vice versa, by overriding LayoutPhase1 and LayoutPhase2 handlers appropriately. For a preview, take a peek in wx/include/base/wx_lay.h, wx/src/base/wx_lay.cc and the layout sample if you have a recent beta.
Eventually the resource editor mentioned in another technical note should support the specification of constraints and sizers. However, don't expect to see it before Christmas 1995!