I think the jury's still out on this one...
On one hand you have FNMI, who says they don't sell anything outside military channels, and that any FN items offered commercially are stolen, rejects or fakes.
On the other hand, you have folks like JT Distributing, who use the FN(R) trademark in their advertising (if they're not FN parts, why hasn't FNMI *nailed their a$$ to the wall?*). And you have Mouseomatic and others who have bought legitimate, genuine FN-marked barreled uppers from various places (gun shows, internet, etc).
I suspect the truth lies somewhere in between... both FNMI and the dealers who sell unmarked uppers as "FN" are probably fudging their stories a bit.
Whatever the truth is, I've built up 3 or 4 guns on these types of uppers and have had no problems. Matter of fact, they've been nicer in color/quality and fit than the Rock River upper I just bought. :-(
A program note - many of the places that advertise "FN" uppers are pairing them with non-chrome-lined, non-milspec barrels from Shaw, Wilson, etc., which is why they're so cheap. A real FN barrel will be chrome-lined, 1x7 twist, and will have an FNMI marking on it.