Can you use your mount as an ally, for flanking purposes, while dismounted?
It's odd that mounts aren't explicitly noted as allies anywhere that I could find.
The Rules Compendium p254 notes that all mounts can attack at -2, although few attack on their own.
Mounts are explicitly targeted separately from their rider (ibid). I find it hard to believe that a DM would not allow the mount to be the target of a beneficial effect like Shield of FaithDDI that only targets allies.
Given that, extending all rights as "allies" to the mounts seems entirely reasonable and non-controversial.
If the mount could attack the foe, then it can count as flanking.
Unless the mount has been trained to fight in this capacity, I would say most likely not. Warhorses are trained to fight while mounted and directed by a rider but typically do not engage in strategic combat and will still flee if left w/o guidance.