Walmart has published an official coupon policy online. Hopefully this is helpful to many of you that have problems at your local stores with coupons.  I would suggest printing it out and carrying it with you when you shop.
A few highlights that I can see:
Walmart will accept:
Competitors’ coupons that feature a specific item for a specified price
for example, $2.99
*So this is great for Walgreens in ad coupons or if your grocery store has in ad coupons (Safeway does all the time!)  The stink part of this is that the policy says one coupon per item so I am thinking they might not take manufacturer coupons on top of those in ad coupons.
Checkout coupons
(also called “Catalinas”) that are printed at our competitors’ registers, have “Manufacturer Coupon” with specific requirements printed on them, a valid remit address for the manufacturer, valid expiration date, and a scannable bar code.
*I don’t think this means Register Rewards.  I think this means coupons that print from the catalina machine for specific products (like .50 off Tide) but it wouldn’t hurt to ask!  I have heard of stores taking Register Rewards with no problems in the past!
On the flip side Walmart will NOT accept:
Print-at-home Internet coupons that require no purchase
*So no free item coupons but you can use a Buy One Get One Free coupon.  Just as long as the coupon requires a purchase it is fine.
And I nearly fainted when I saw this part:
Coupons may exceed the price of the item. Change may be given or applied toward the cost of the basket purchase.
*Hello overage!  What that means is that if you find an item for $1.00 and you have a $2 coupon, you will get the full value of the coupon credited to your purchase OR they will give you $1 back in cash!  I have never seen that clause in any coupon policy ever and I am very curious to see how this actually works out for you guys.
You can see the Walmart coupon policy here.
(Thanks Shelly and Coupon Cravings!)