Engineers won't need Volatile Air anymore

Well, actually they will still need some [Volatile Air], but a lot less than now.
Look at this:

• The [Electrified Ether] recipe now creates 2 to 3 Electrified Ether instead of 1.

What is [Electrified Ether]?
Well, it actually is the way Engineers use [Volatile Air]: they transform their [Volatile Air] in[Electrified Ether] at an exchange ratio of 2:1 (for 2x[Volatile Air] they get 1x[Electrified Ether]) and than they use the [Electrified Ether] in their recipes... A lot of them.
Actually, most of their recipes need at least 2x[Electrified Ether]. Right now they need:

•  6x[Electrified Ether] for each epic-quality helm (but still, you make just one in your life, two if you want one specifically for your off-spec)
• 8x[Electrified Ether] for each of the new Engineering pets ([De-Weaponized Mechanical Companion] and [Personal World Destroyer])
• 8x[Electrified Ether] for the ilvl 333 trinket, [Elementium Dragonling]
• 8x[Electrified Ether] for the ilvl 346 gun [Overpowered Chicken Splitter]r
• 2x[Electrified Ether] for the ilvl 333 gun and crossbow ([Volatile Thunderstick], [Finely-Tuned Throat Needler])

Also the [Loot-A-Rang] and the [High-Powered Bolt Gun] need some [Electrified Ether] (5x and 4x respectively), but I don't know how many of these are being made.

Anyway, it seems to me that [Volatile Air] were needed for a lot of things, and now for the same things Engineers will need less than an half of them.
As this wasn't enough, the PTR notes also say:

• [Volatile Air] now has an increased chance of dropping from rich mining nodes and Pyrite.

Changes in prices
I'm not sure about how much this will affect the price of [Volatile Air], this depends heavily from how great will be the buff at their drop from rich mining nodes and nodes of Pyrite. Even with the changes made at Engineering there still are lots of untouched professions that will still need [Volatile Air]... Anyway, I'm quite sure we'll see a drop in price once 4.0.6 will hit the live servers.
What I can say for sure, though is that the prices for the Engineering pets, De-Weaponized Mechanical Companion and Personal World Destroyer, will go down by at least 320g, requiring 8-12 x Volatile Air less than now.

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Other engineering changes
It makes sense to quickly comment the other changes that will probably (remember: all of this is still just for the PTR) be made to Engineering:

• The [Heat-Treated Spinning Lure] buff duration has been increased to 20 minutes, up from 5. In addition, the recipe now creates 2 at a time.
• [Synapse Springs] now increase Agility, Strength, or Intellect (whichever is highest for the character).

The first is more a buff to fishing than to Engineering, but surely a pleasant one for a lot of people. I don't know how will this sell, but since the amount created is doubled they could become a really cheap thing to craft you could be able to sell for some money.
The change to [Synapse Springs] makes them useful for everyone and not just for spellcasters. Will this lower the price on other gloves enchants? Well, I con't think you should be afraid of the lowering of the demand for gloves enchants: lots of people still don't have engineering on their main (I'm one of those, actually) and this isn't even something you can learn from a trainer! [Synapse Springs] works very similar to Alchemy discoveries: you can learn it randomly while crafting other Cataclysm Engineering items.
It's even possible that some of the less-informed engineers out there don't know about this and aren't trying to get it, since this isn't such a usual mechanic for engineering.

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Other posts about the PTR 4.0.6 patch notes:

• New metagems on the way!
• Sockets will be added on crafted epic gear
• New enchanting recipes (and nerf to [Enchant Off-Hand - Superior Intellect] )