never seen this problem but I don't sit in the seat while the tailgate is open either... though today I was just picking up the wife and as she got in, it said my key was not detected... I was sitting there the whole time with it in my pocket and the engine running. It freaks from time to time, but it goes away quickly... nothing like what you're seeing.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr