Re: 'cuts' in the treads in the tops of plastic soda bottles.

I didn't know the answer to this one, so I sent an e-mail to Alcoa's 
Closure division (I figured that Alcoa knows about anything associated with 
aluminum, including lids).
I received the following answer:
      "In response to your question, the thread "cuts" or "gaps" in the
threads on both the bottle and closure are vent slots which allow the CO2
gas to escape from the inside the package during opening. This escape of 
gas
relieves the pressure on the inside of the bottle and allows the closure to
be safely removed from the package by the consumer. The reason these 
threads
are only found on soda bottles is because soda packages are normally the
only packages pressurized with CO2. Non-pressurized packages (like bottled
water) normally contain smooth, non-vented threads since there is no
internal pressure to be relieved from the inside of the package. Please
advise if you have other questions."

      Tony Smith
      Alcoa CSI

I wondered whether beer bottles didn't have the cuts in the lids because 
they are under lower pressure.  Again, Mr. Smith responded:
     "The internal pressures in a beer package are much less - the CO2
content of beer is roughly half the CO2 content of a carbonated soft drink.
It also depends very much on the type of bottle and closure system being
used for the beer package. In a traditional longneck beer bottle with a
steel crown, the opening device bends the crown while holding it in place,
which effectively vents the container safely. If the beer package contains 
a screw on closure, the bottles and closures are sometimes vented and
sometimes not - it really depends on the preference of the company. Vented
threads are not required at all for packages with this low a level of CO2,
but some folks like to use them anyway. 

  Tony Smith

So there you have it.  The vents are intended to relieve pressure as a cap 
is removed.  You are a pretty keen observer to have noticed them.  And 
thanks to Mr. Smith for the information.