I want to believe that, must if it isn't true? If I buy a new 2600€ rMBP now and in September / October another model comes out?
Apple may give the MacBook Pro's a minor Haswell CPU refresh in Q4, but if you need a new computer now, then buy now (be sure to avoid buying directly from Apple, Amazon undercut Apple by several 100 for standard config machines). If you don't need a computer now then just wait for the Broadwell refresh.
Broadwell will not greatly benefit the MacBook Pro's anyway aside from slightly better performance, slighly better power efficiency & maybe a 10% boost with integrated graphics. In day to day tasks nearly everyone wouldn't be able to tell the difference between a Haswell or Broadwell based machine.
Broadwell CPU's will greatly change tablets & Ultrabooks though, with the Y series CPU's. They are not powerhouse processors and probably won't compete with ULV processors in the current MBA when it comes to performance, but when it comes to all day battery life, they will be a game changer.
If Broadwell is merely the "Tock" this time around. Skylake is what should excite people & the industry.