local x, y = GetPlayerMapPosition("player") seems to return valid coordinates in instances now. Note that this only seems to work inside Northrend instances. It will still return zeros in lets say Eastern Kingdoms (tested in The Stockade) or Outland (tested in Auchindoun).
I thought I should wait a bit, but might as well post it now. It would appear that addons using Blizzard's dropdown menu system will error if they are initializing a menu containing more than 2 levels, once the 3rd level (listframe) needs to be shown. Simply put the dropdown code does not create a listframe with a level of 3 or above (e.g DropDownList3), which would imply that the UIDropDownMenu_CreateFrames(level, index) function isn't being called properly (it should be normally called when [FONT=monospace]the [/FONT]secure? attribute "createframes" is being set). I first noticed this in Postal but seems to be a generic issue.
As a workaround, you can call the CreateFrames function in your addon directly (let's say after performing a check, using UIDROPDOWNMENU_MAXLEVELS) with the desired level (>2) variable and from that point on, things seem to work fine, however you should not be doing that in the first place.