Which reminds me... this got bumped today.
astronomy.stackexchange.com/a/19241/16685 I suppose Dewi is thinking that the Shell Theorem (& Birkhoff's in GR) prevents a homogeneous shell from collapsing. That sounds quite wrong to me, but I'm not having much luck finding a nice clear & simple online ref, although I see plenty of papers talking about the collapse of a thin shell, particularly in the context of black hole formation.