Thanks petrified.rabbit:

what kind of plugs did you put in? check them again?
I put in regular NGK V-Power - not iridium. My 2005 had stock regular AC Delco's, not the Delco Iridiums recommended in the owner's manual. The old plugs had badly worn central electrodes, but all wear was uniform. All had proper coloration (light tan to brown). I did check them again. I will have to check my notes as to when, but I would guess 5 to six months after install when mileage started to go south again. The new plugs had good electrodes, but all of the plugs had a black sooty color. I checked them again later (after the car had run awhile after a commute) and the color was better. I'll check them again tonight. I was definately thinking too rich of a fuel mixture when I saw those sooty plugs.

Note again that I have a K&N drop in filter that was installed April 2009 and cleaned in November 2009 (just before winter).

you could check the o2, iat, and map sensor signals.. those could be off
but only the o2 would have potential to adjust back with a plug change, but going bad again that quickly? not sure about that..
I haven't tested the O2 Sensors. I don't have a scanner and I haven't checked for codes. The CEL has never come on. That said, I plan on testing the O2 sensors using a multimeter using a onboard/in situ method.

I believe I have tested the IAT with a multimeter. I had a good "How To", but I didn't have a volts to temperature reference to use for a conclusive test. All I could measure was whether there was a "reaction" and the reaction rate in the IAT. I assumed the IAT is the only sensor on the air intake tube (after the air cleaner box).

I couldn't find a MAF sensor on the Aveo. If there is one, let me know where and how to test it.

Also note that my car has had the "cold weather intake" recall/upgrade done to it. This was to prevent potential icing of the throttle body.

How many miles were on the old plugs? were the 4 uniform in their condition? did you replace the wires or check the coil pack?
Assuming they were never changed, about 70,000 km (43,750 miles). The new ones have about 25,000 km. I did do a resistence check on the spark plug wires and they were consistent to each other. I'll have to see in my notes what the resistence was. I did not test the coil pack. How would I do that?

Would a compression test be they way to find bent/leaky valves? How would I test for a plugged CAT, other than smell? My understanding is that a bad CAT would smell like rotten eggs all of the time, no? Would the following be advisable?

"When a clogged converter is suspected, some mechanics temporarily remove the O2 sensor from the exhaust pipe ahead of the catalytic converter and look for a change in performance."

Thanks!