Alright, I couldn't wait til I got home. Wrote a quick dirt little app and deployed it to my Lumia 900 and the DeviceStatus.DeviceTotalMemory property does indeed report back the total amount of RAM that is avaliable for all non-OS apps. Microsoft also has this documented on MSDN (under "Remarks"):
DeviceStatus.DeviceTotalMemory Property (Microsoft.Phone.Info)
For all you developers out there, I did this as an example:
txtBoxDeviceTotalMemory.Text = (Microsoft.Phone.Info.DeviceStatus.DeviceTotalMemory / 1048576).ToString() + " MB" ;
The API names and the label names on the apps are misleading, but rest assured that your Lumia 900 does have 512MB total physical RAM. So move along now, there's nothing else to see here.