Just to add another voice that the Brother HL-2040 is not working correctly with DSM 4.1-2647.
In my case its with a DS1512+ .
Same symptoms as other posters:
* iOS sees there is an airprint printer available (usbprinter1)
* when printing using airprint nothing happens; no lights change on printer, nothing
* using lpstat on the command line and I can't see any jobs being submitted to the queues
* printing from Windows client works