As a mount can be a pricey investment that can be literally chopped out from under a character, these questions came to mind.
Does a mount count as an ally for item use?
Yes. They are willing creatures.
- With most mounts only having a single healing surge (or would it be 2 like a beastmaster's animal companion?), would they be able to have an item like the Healing Sash used on them?
Yes.
What all items can a mount wear at once?
- I figure Armor slot (Barding), Head slot (Bridle), Waist slot (Saddle), Foot slot (Horseshoes/Greaves). Would they also be able to benefit from a neck Slot (Amulet/Collar/Caparison)? Some mounts are bipedal with arms and hands (especially the reptiles in Eberron), so would they be able to use Arm slot (Bracers), Hand slot (Gauntlets/Gloves), Finger slot (Ring)? Can Mounts have a magic tattoo? Since all the mount items with powers in AV1 are activated by the rider, this should work with any other slots as well right?
Mounts can wear a single Mount category item (barding, bridle, horseshoes, etc.). See Adventurer's Vault. Companions can use a Companion or Mount item.
I do not think they can wear anything else although item effects from PCs can still affect them. This is more implied than explicitly stated. For example:
Although mounts are mainly used as basic transportation, others (especially warhorses and other combat-trained mounts) might take advantage of specialized gear such as barding (described below) and mount magic items (described in Chapter 2).
...
A creature serving as your companion or mount has a single magic item slot that you can activate using your own actions (and not the creature’s).
Regardless of whether they can use items outside of Mount items, they are still limited to one slot for magic items. They also have multiple slots for non-magical items such as barding.
Since the Mount magic items include bridles and barding, etc., I do not think that one can claim that they can use waist items or armor. In fact, there is magic barding rule that prohibits magic armor enhancements in barding (but not magic armor effects which can be transferred).
When can a mount use a Healing Surge?
- Is it the same as anything else, like using 2nd wind in combat or after a short or extended rest? I seem to remember reading somewhere about a mount being to able to use a HS when its rider uses one, or a rider can use a HS on its mount when he does or something like that? But I can't find it now. Its totally possible that I'm remembering something from a completely different system ;p
Mounts are monsters (for all intents and purposes). They cannot use a healing surge unless a power or effect allows them to. For example, a heroic level mount can be healed as an ally once per day by a Cleric.
Technically, they are allies (i.e. willing) and they can be considered NPCs (there is no rule to prevent them from being NPCs), so they can have a Healing check for Grant a Saving Throw. But, a PC cannot use a Healing check for Second Wind on the mount because the mount does not have Second Wind to begin with. And Stabilize the Dying doesn't work because the mount is typically dead at zero hit points (shy of some item or some such).
Does a mount get Opportunity Attacks?
- I would presume they do since their space is larger than the riders. For instance, since the erratta where you have to choose which square inside the mounts space the rider is in, it leaves squares where the rider can't reach that the mount still threatens. So would it be a joint set of actions type thing, where if the rider can't make the OA then the mount can as long as its still only once per opponent's turn? Or would the rider just shift to any square in the mount's space and make all the OAs?
First off, one does not choose a square for the rider in the mount's space. The rider is technically in all of the squares of the mount. Choosing the square is only for origin point for attacks (such as melee, close bursts or blasts, or ranged). The "if it is important" rule from the DMG has been overwritten and dropped in the Update.
A mount is a creature like any other creature and can take Opportunity Attacks. However, the shared action rules have been updated to state that there is only one OA that either the rider or the mount takes.
Remember, flying mounts do not get OAs unless they have Hover.
Does a mount get Action Points?
Not unless the mount creature type does which I don't think ever occurs.
Unless the mount gets a separate set of daily item power uses. Then it might be worth exploring 
