I have taken the BSDA in 2012 in Melbourne (expired this year).
Actually it covers all *BSD family though I don't think it really matters much.
Some usual networking-TCP/IP stuff, subnetting, kown assigned port services etc.
But there's no routing/switching as hard as in some vendor specific certifications.
I think there's also some question about BSD ports/package management.
Of course they'll be new material since then, but I think if you've been using FreeBSD for a year you'll be fine.
Anyway, to support FreeBSD, you should get certified and buy the study CD
I've done a lot of these. At one point in time I held (if I remember right) I had to renew 13 exams to maintain my certifications. I've let them all expire now though. But decided to pick up the torch and try my hand at Juniper. Love their stuff.
I think I will give a BSD exam a try someday in the not so distance future. Even if I fail the exam it will surely help me learn the various topics of BSD.