1.8.1b540 supports usb 3g cards etc. for WAN. hardware support is whatever is in freebsd 8.3
newer sticks require a custom build, you may just have to add the new usb id's as huwaei generally uses the same drivers, but they may not be associated as the id's may not be known.
whats supported is the same as the list you will find here
http://svnweb.freebsd.org/base/release/8.3.0/sys/dev/usb/serial/u3g.c?view=markupE3131 it appears to have some support in the just released 8.4, so we may see it in m0n0wall soon