Well, I ended up calibrating the TV with the Avia disc and, while the overall picture looks slightly better, the halo/ringing effect is still there. However, it is not quite as obnoxious as it was pre-calibration. Still noticeable though.
Also, just for kicks, I hooked up an Xbox via component cables. It's not the greatest quality DVD player, but I was curious to see if the same haloing effect would exist, and it does. So, based on that, it seems the TV itself is the source of the problem. At this point, I guess I am willing to live with it. Although, it didn't stop me from checking out the Sony Grand Wega 50" LCD rear projection set at various stores this week.

Nice TV, but I can't justify dropping almost $5000 on a new TV after only owning the 36XBR400 for a year and three months.
The good thing is that the problem is not my Sony 330 DVD player. It's been an excellent workhorse player for me and it would have been a shame to have to replace it.