Was looking around for aftermarket air filters and I found that K&N has added the x1 to their database. I guess they are just the standard parts used in the 1 and 3 series:
2010 BMW X1 2.0L L4
http://www.knfilters.com/search/prod...px?Prod=E-2021
2010 BMW X1 3.0L L6 F/I
http://www.knfilters.com/search/prod...px?Prod=E-2022
2010 BMW X1 2.0L L4 DSL
http://www.knfilters.com/search/prod...x?Prod=33-2942
Just in case anyone else is looking.