Sepherosa Ziehau has added a preference for the emx(4) driver over em(4). This means that loading the emx module or adding the emx device to your kernel config will may turn your em devices into emx. This isn’t dangerous; just a note to keep it from being a surprise.
Edit: Thanks, Sascha Wildner, for pointing out details on which chipsets are affected.
That’s not correct. It will override _certain_ em(4) devices, as noted in emx(4)’s commit message:
http://leaf.dragonflybsd.org/mailarchive/commits/2009-03/msg00007.html