I've managed to get suitable stand offs, feet and Nylon bolts for a Rev2 Pi! I've also fitted P5(On TOP) & P6 and can confirm you can plug in a GPIO cable and an RTC at the same time, though the DSI connector is obscured but was also obscured if the RTC is fitted to P1, though by not quite as much.
I mounted the Pi on a slimline CD Jewel case lid so you can see through it, unfortunately on the photo of the underside its so clear it doesn't seem to be there!
 The pin connections on our Real Time Clock will need modifying to work on P5 but the board will be the same size.
We will be stocking suitable spacers, feet, nylon bolts, P5&6 pins etc very soon. They will appear on our RPi page at: http://www.cjemicros.co.uk/rpi