Basement Return Location
Local codes will differ from area to area, if you are doing the reno yourself it probably won't make a lot of matter. If you are concerned call your local building inspector or gas provider. Typically a floor location in the basement is nice because it takes the cold air off of the floor to recondition.
The only place you really don't want to place one is in a room that doesn't have a supply duct as well, and in any room that contains a fuel burning piece of equipment, like your furnace room, or one with a fireplace or gas stove etc. (The danger here is that you could cause a negative pressure area and draw combustion products back down the chimeny system and cause possible carbon monoxide poisoning.)
Hope that helps,