The best solution is to replace the whole windshield, this is what BMW service center's solution. However, you can buy the BMW specific glue to re-attach the sensor onto the windshield, but it will be really difficult to do it without letting air goes into the area as long as the windshield is not off.

__________________
--2004 E53 X5 4.8is - ECE version; right hand drive; Located in Hong Kong; ext. black sapphire metallic; int. leather black; adaptive dynamic bi-xenon headlights; 16:9 onboard monitor w/o navigation; 20" genuine BMW Y-spoke 214 wheels with Bridgestone Dueler H/P Sport tires; Int. rear view mirror w/ compass.