The metal oxide coating California's "Cool Cars" regulation mandates for car windows starting in 2010 doesn't just reflect sunlight—it also bounces the signals for cellphones, GPS navigators and, um, parolees' ankle bracelets, making reception noticeably crappier.Updated.
Which, I suppose, is one way to kill two birds with a single government-tossed stone: people won't be texting while driving because they can't get a decent signal anywhere, and cars will have a smaller carbon footprint since they won't get as hot. The whole GPS navigators sucking and not being able to accurately track parolees is a different problem for a different day, really.
The car manufacturers, for their part, would prefer a coating that absorbs energy, instead of reflects it, which they say would leave wireless signals unmangled, but I bet slightly warmer.
Update: California's Air Resources Board says your phone will be fine in this FAQ (PDF):
Specifically, ARB staff conducted a test program in Southern California involving three cars, one with reflective glass all-around, one with reflective glass only on the windshield, and one with no reflective glass at all. The study showed that cell phone performance was equivalent, regardless of how much reflective glass was used.