I had the same issue at around 40k miles. Just take your car to the dealer and they'll replace the front passenger seat coushion and sensor for you. There wasn't even a recall going on when it happened to me but they didn't hesitate to replace it.
To respond to the problem, BMW said it would extend the warranty to 10 years without any mileage limit for the following vehicles: 2006 6 Series, 2006 3 Series with standard seat, 2004-2005 Z4, 2004-2006 7 Series, 2006 X5, and 2004-2006 5 Series with comfort seats.
I've got sport seats and still had the problem but I assume they would also cover sport seats if you're out of warranty on you 325 and you have sport instead of standard seats.