Not sure about foreign airlines, but I have never, ever seen a US airline make good on their "best fare guarantee." Caveat emptor.
Fti, you were SO right... Unfortunately I did not follow your advice, and learned the hard way. A mistake which cost me more than eighteen hundred dollars!
Hope others can learn from this and avoid the LFG ploy at Brussels Airlines.
Long story short. I printed out all the fine print of their "guarantee" and sat with a few friends and family to study the t&c thoroughly. After we were sure that we got each and every point on there, I went ahead and purchased through them.
Turns out the whole "guarantee" is, what appears to me as, a ploy!
I had a friend write up about it and post it to a blog he created. It's too long to copy and post here, but check it out:
http://snba-scam.blogspot.com/
I personally will never fly with them again, and neither will anyone of my friends and family.