I based my reply on the "Owner's manual video" about X5 TPM on the BMW USA site. You know the one that I downloaded and viewed while I was astonishingly bored waiting for our 2009 X5 35d to be built. Maybe it changed for 2010. I'm downloading the 2010 version now.

Funf Dreisig

Edit: The 2010 version of the X5 Tire Pressure Monitoring system still says the system measures the relative speed of the wheels and makes no mention of any direct pressure measuring. Of course, we know that BMW USA's site is not always up to date. So this video may be wrong.

FWIW the 2009 Owner's manual says that you should "always use wheels with TPM electronics".