I found that if you try to run both usb0 and mwlan0 udhcpc will cause one or both interfaces to stop working.
The line in /usr/share/udhcpc/default.script that looks like:

   ip route flush dev $interface

is the culprit.  Commenting out that line will allow both interfaces to work.  I'm using version 1161.
Someone ran into this problem a while back, but I can't locate the message.

There is also a problem with udhcpc and the bootpd on OS X.   It looks like udhcpc sends a broadcast packet with
a length of 590.  For some reason bootpd doesn't like the extra padding and doesn't respond.  I'll have a go at fixing