Have the designers mentioned anything about mounted combat in 4e? I hope its on the list.
There are rules for it in the DMG and player options for it in the PH. It's in the DMG because DMs have the option of giving out mounts and using them as extra party members (like in 3e), or using rules similar to the mount rules in the latest set of D&D Minis.
We also allow the option for DMs to mix the two: mounts that are low intelligence or that don't normally attack in melee use rules similar to DDM, a creature like a dragon uses rules similar to 3e (it's effectively an extra PC).