FYI:
I have found a LPG convertion kit that will let you select gas or lpg .
The kit comes with a 14 or 16 gal tank that sits where the spare tire is.
There is a control unit in the cab that lets you switch from gas to lpg at a push of a button.
This control unit also monitors the lpg gas level in the tank.
The ECU can be interfaced and tweak if needed.
The kit it about $900 and the tank is about $600. Propane is everywhere! just about every gas staion has a tank, 7-11 , quick stops , tool rentals and the list goes on...
I am looking for a certified company here in CA that will do the install for me , You could do it yourself , but it has to be installed by a certified installer in ordered to be driven on CA hiways.
The only real hard part of the install is drilling the holes in the intake manual for the lpg injectors.
I think I have found on certified dealer called Bay Tech that will do this, but no call back from them yet.
With the current price of gas, I think this will be a great investment.

Here is the link to the product I am looking into...
http://poweredbypropane.net/

I am out of state and dont have the email with the tank infor, I will get that when I get back home and put it in this message.
The LPG tank looks like a spare tire and sits in the spare tire bay.
It comes in two sizes, 14 gal and 16 gal.
I will also put a link to an X5 I found in the UK converted with pictures (not the same kit ,but shows the install and the same type of tank).
Here is a link with different BMW's converted to LPG: http://www.arunautogas.co.uk/lpg_conversion_bmw.htm
http://www.gastech.org.uk/installgallery/bmw_x5.html

Here is a little update>>>
I decided to sell the X5 and get a new Z4 and convert the Z4 over to LPG. Could take a few months to do...