  The dark matter dominated Fornax dwarf spheroidal has five globular clusters
orbiting at ~1 kpc from its centre. In a cuspy CDM halo the globulars would
sink to the centre from their current positions within a few Gyrs, presenting a
puzzle as to why they survive undigested at the present epoch. We show that a
solution to this timing problem is to adopt a cored dark matter halo. We use
numerical simulations and analytic calculations to show that, under these
conditions, the sinking time becomes many Hubble times; the globulars
effectively stall at the dark matter core radius. We conclude that the Fornax
dwarf spheroidal has a shallow inner density profile with a core radius
constrained by the observed positions of its globular clusters. If the phase
space density of the core is primordial then it implies a warm dark matter
particle and gives an upper limit to its mass of ~0.5 keV, consistent with that
required to significantly alleviate the substructure problem.
