Since its under warranty I wouldn't worry too much. Yes it shouldn't happen but a valve cover gasket is not a big deal mechanically and shouldn't have done any damage unless you ran it low on oil. Any possibility it was ever overfilled with oil? I 've never heard of BMW's having this issue so I think it's a freak occurrence. I assume its the N20 engine, I'd search the 5 series and 3 series threads to see if this has happened to anyone else.
1995 325i,1996 328ci,1997 528i,1997 Z3 2.8,
2001 X5 3.0,2001 330i Conv,2002 M3 Conv,
2003 M3 Coupe,2004 M3 Coupe,2004 Z4 3.0,
2004 X3 3.0,2007 X3 3.0,2007 335i Conv.,
2013 X1 28 sdrive M Sport, 2014 M235 on order