I think it's a great idea. I think you could perhaps reword it to make things a bit less weird sounding though: blocks are created using curly brackets, but correct indentation is enforced by the compiler. Indenting weirdly (e.g. mixing spaces/tabs) will cause a compile error.
Also I can't see how it would affect code sharing at all. I assume it is per-file, so adding entire files won't matter, and if you have a project that wants to steal a bit of code from another in one file, surely you want to convert their indentation to yours - which would be trivial to do given that you enforce correct indentation.
I think it is a good idea. If you ban tabs you'll piss off the sane 50% of programmers, and if you ban spaces you'll piss off the other 50%. Allowing either but with no mixing makes sense.
The only exception I can think of is that some people like to do tabs for indentation and spaces for alignment. Dunno if you could support that easily...