And hey, maybe it is true. Maybe that type of plane would make the perfect weapon. That doesn't mean this is what's happened to it, and in fact it would be strange if it was. A lot of terror groups are very well funded and well connected - they wouldn't need to steal a plane full of passengers, thus putting every security force in the world on alert, in order to get their hands on a plane.
B777-200/300 in a working condition with resource left to fly before next c-check/heavy maintenance and working engines starts at 40 million dollars. You can get one for under 40Mils but it will require heavy maintenance to make it air worthy. If it is not airworthy then one would not be able to conduct even the initial stages of what is to be a future "illegal flight/operation".
Terrorist groups maybe heavy funded and be able to afford this sum, but it is not penny-purchase, and the expense must justify a rather major purpose.
But even if funds were available, it is hard to buy a Boeing and hide the transaction, or even be allowed to complete such transaction.
Major aircraft sales/purchases, leases are hard to hide, Boeing for example monitors such transactions; there is a whole scope of aviation equipment produced in USA that can not be sold to/in certain countries. Such transactions are heavily monitored inside and outside the USA. Any person or company which sells/buys (for example) Boeing A/C and equipment by Boeing or Boeing approved OEMs/parts/avionics has to fill out extensive paperwork and go through approvals. It is not a simple procedure. The entities/airlines/people have to be VERY legitimate and for the lack of better words, very clean and with long professional history in the field of aviation, and with proper registrations for airlines.
Any intermediary who would be willing to act as a legitimate entity to lease/buy such A/C, to later turn over to "questionable entity" is basically "dead meat" once the A/C is used for its intended illegal purpose. It would be hard to believe that ANY registered airlines (with IATA, local aviation authorities like FAA), or leasing/sales brokers, or individuals working for airlines or brokers, who passed the filters of being allowed to sell/buy Boeing equipment would be willing to loose everything over 1 sale or lease to "questionable buyers/lessees".
The to take possession of the A/C the lessee must "move it" to the desired destination, called either a "ferry flight" or "positioning flight". Just that transaction alone requires a bunch of registrations procedures in which the country of registration must be stated and bunch of airworthiness certificates.
Even in banana countries where there is less control, still certain registration procedures must be followed, because Boeing monitors the activities and movements of their equipment (based on the US demand that the equipment can not be sold in certain countries). That is not because Boeing strongly holds to political ideologies, although it might, but because it could be fined by US government....
http://www.nytimes.com/2006/04/10/business/10boeing.html?_r=0
So, you can't really take a functioning Boeing equipment, wheel/deal the sale, put it in your purse and walk off with it..

It is much easier to pinch one even with all the troubles and risks.