Alright, the results and answers are in:
We asked several ECN brokers including MB trading to explain "How one can verify the claims of a broker being an STP/ECN."

MB Trading replied:
"MB Trading Futures maintains orders anonymously using STP/ECN technology. The technology itself is modeled after the securities industry and facilitates best execution for you and generates price competition, which means traders can "skip the middle man" and save money, rather then using other traditional FCM’s using deal desk models. The firm accomplishes this by routing orders to banks and pools of liquidity to create a true “non-dealing” desk platform. Traders have the ability to route orders via fifteen order types to qualified destinations through our STP (Straight Through Processing) platform.

Since there is no central marketplace for forex, there is no website/regulatory group similar to that of securities for you to verify. The only thing I can suggest is to perhaps try out our demo so you can experience the ECN technology yourself to see the differences between deal desk methodologies and our model."

NFA-regulated brokers would have to meet $20 000 000 net capital requirements by May 19th, 2009.
According to latest data released by CFTC, MB Trading had $17,031,724 of net capital as of March 31, 2009.
Most likely MB Trading will get the required capital closer to the deadline.
I'd recommend you wait till May 19th before making any moves.

I trade news with MBT (ECN), they are rather good, but recently my pending orders were filled with totally different price than requested. Client support said that its due to lack of liquidity...