Now here's a clever idea for an app: using the position sensors in your iPhone, the free RoomScan app creates a pretty accurate floor plan (within a foot or so) of any room.
When you launch the app you'll see some help screens, and there are links to several videos showing RoomScan in operation. To get started, you just touch your iPhone to any wall. Walk to another wall and tap the iPhone on the wall, and then go to another. When you have touched all the walls, go back and touch your original wall, then tap the 'finish' button. RoomScan then draws a reasonably accurate floor plan. The app guides you by voice in what to do, and it if feels it doesn't have accurate data it will ask you to repeat the process. If you bring your iPhone or other iOS device to to the wall with a jolt, RoomScan says "ouch!" and asks you to try again more gently. You also need to move fairly quickly between walls, or RoomScan asks you to pick up the pace.