Precisely, and this proposal is intended to keep that aircraft (A340-600) from getting any more successful. As discussed in the Orders forum, this aircraft is initially targeted for two specific customers: Emirates Airlines and Cathay Pacific. EK has a specific performance requirement that both Boeing and Airbus are trying to match. At the moment, however, neither aircraft (773ER & A346) can do all EK want them to do, which is why they have been considering both aircraft (@ 8 A346s & 25 773ERs). However, if Boeing can get the -250ERX to match the payload/range requirements of EK, then they could potentially secure both orders, which I've already heard could easily be increased. We would therefore be looking at an EK order alone worth about 35-45 frames.
As far as CX goes, they are in a very similar position. They are going to lease 3 A346s from ILFC on a trial basis, to see if the aircraft can meet certain performance goals on some of their more lucrative routes. However, they have yet to make a desicion on a larger order. As Alain has already pointed out, CX's initial displeasure with GE's sole-supplier status has pretty much evaporated in response to promised performance goals and maintenance packages from GE. If the -250ERX can beat the -600 on payload/range stats, then it is possible CX could also order 10-15 of these aircraft, as operating costs would naturally be lower. Suddenly we have 2 customers worth nearly 60 orders.
As already mentioned by Alain, the agreement between Boeing and GE covers any 777 derivative with a MTOW of 700,000+ lbs. At the moment, this only covers the -200LR and -300ER. However, any derivative over that weight is covered, which would include the proposed -250ERX, any freighter based on the -200LR (which has been rumored), or the fanciful -300X Stretch.
Along those same lines, BA once approached Boeing about building a -300 with an MTOW hike, but without reaching that 700,000lb. limit. That way, BA could still order the Trent 800 for the "-300 Heavy" (or "-300ER Lite", however you want to look at it).
All gave some. Some gave all.