From what I can tell, all of the retail units they're having problems with were manufactured on August 12 or after (console manufacture date). Drive manufacture date is apparently irrelevant, the change seems to coincide with the date of assembly, which makes sense as that's when the drives get locked to their motherboards. This means it's probably not a change in firmware (unless they're flashing them on-the-fly, which IS a possibility), but the encryption key used to encrypt the drive key. In other words, I'm suggesting there's no firmware change because the firmware supports layered encryption similar to the scheme used in Blu-Ray. The decrypted drive key being obtained by somehow mixing the encryption key with the encrypted drive key using an algorithm that doesn't change. Find the algorithm, and it's all over.
If you want to get one you can be sure will flash, make sure it was manufactured in July and does not come with 2 games. You'll probably be fine if you find one from May or June but the further back you go the higher your risk of getting a 7 series drive (and therefore having to order a probe). The chance of a 7 being in a July console are extremely slim. I bought an elite with a July 31 MFG date and was able to flash it fine (I put Talismoon replacement fans in as well since I had the case open anyway, it's a Jasper, but better air circulation never hurts). So far there are no reports of a console that was manufactured in July that could not be flashed (though there are plenty of reports about drives from July in consoles manufactured on or after August 12 not flashing). And as you just read, mine was made on the last day of the month and still worked. There are some reports of August consoles made prior to the 12th (some from August 5 for example) working, but it's best to play it safe. Don't panic if you don't find one at Wal-Mart. Pick a place that draws smaller crowds like K-Mart, Best Buy, or Toys R Us. You'll find there's plenty of July units still on shelves.
For the record, I used a via 6421 card, JungleFlasher 1.6.6, and iXtreme 1.6 firmware. I highly recommend JF. If you follow the instructions in the PDF it's nearly foolproof. And that's what you want considering one missed or improperly performed step could turn your console into a doorstop. If you're using a 64-bit version of Windows, you'll need to reboot and pick your 32-bit Windows from your boot manager (what I did), start a VM that's running a 32-bit Windows, or make a Bart's PE stick drive using a 32-bit Windows disc. You really do want to use JF unless you're extremely comfortable with the DOS prompt (I am and I still went with JF).