I just got a '03 X5. Had it about 2 weeks, it's awesome. Yesterday I was driving it in some pretty heavy rain and got the dreaded "transmission fail safe program". I immediately thought it was due to a electrical issue, prolly wet wire connection. I searched: trans fail safe in rain . I saw several posts about this issue. The most liklely suspect is the TCU has gotten wet. I opened up the e-box and saw standing water in the unused cavity of the control. I took the control out of the car and took it inside and opened up the box to expose the circuit board. Sure enough, I could see water has been getting in to the circuitry by the mild corrosion and stains. I shook it and a bunch of water droplets flew out of it. Then I took a hair dryer to it for 5min. I sealed up the case with electrical tape as a preventative measure. Put the control back in, started her up, and lo and behold : no more fault. Drove her around the block as the theme song from the movie "Rocky" played in my head. Thank's xoutpost .com. The posts in this forum helped me through this issue.
I have had several BMW's in the past. The one before the x5 was a '88 750il with the m70 v-12. So I'm no stranger to "the BMW way" and working on these cars. Here are some pics I took.

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