Have I got some bad news for you guys.
In our case it was a book which pressed back on the recline button as my wife got out of the car. Seven hours later we found the seat back mashed against the bulkhead in the Z4 and the electric motor spinning free.
After taking the seat out (make sure that you don't turn the key on while the seat is disconnected from the wiring harness or you'll trigger a fault in the airbag system which requires a code reader/resetter or a trip to the dealer to clear) and trying all kinds of things to get the seat back to move, I had to pry the seat base apart to get the reduction gears for the seat back out of it. This gearcase is not meant to be taken apart, but vicegrips are amazing! I found out that one of the plastic gears had broken. The reason that the seat back wouldn't move is the amount of reduction that's going on, you just can't make the gears turn the other way. (think about putting your bicycle in its top gear and trying to start from a stop)
Now comes the bad part. For the Z4 (and I'd assume the same for other BMWs) the seat base is one part number (with the exception of the electric motors). In Canada that part for the '03 Z4 is $1400.00. In the US is was somewhat better at $850.00. I found a used seat base at a wrecker for $350.00. I had to take the reduction gearcase out of the seat base I bought and put it into my seat. It wasn't an easy process - lots of cursing and a skinned knuckle.
Sorry for the bad news, but it is something that you can tackle on your own. If not, you'll likely have to find an independant who'd be willing to tie up a tech for a couple of hours to do the work. I'm sure that a BMW dealership would want to just replace the whole seat base.