I heard of the same issues with the 128i/N52, so a couple weeks ago, I changed my idler pulley, tensioner pulley, and serpentine belt as my car is approaching 60k miles.
While it's admittedly a plug for an ad I posted here, I also wanted to flag for others reading these posts that I have new in-the-box OEM tensioner pulleys for sale for less than the retail price here at this link:
http://www.1addicts.com/forums/showthread.php?t=1463178
An order mistake resulted in me getting more pulleys than the one I intended to order for the swap I mentioned earlier. I'd merely like to get some money out of them but also clear them off my parts shelf. Thanks.