Can someone clear this up for me?

I am looking on 2filter.com's website and in the FAQ under Circular Polarizer reads this:
Circular Polarizer: (all mfg.) A polarizer designed for cameras with "beam-splitting" metering systems and auto focus systems. Dslr cameras use a beam splitting prism in the light path to direct the light to the meter. Beam splitters polarize light. If a conventional polarizing filter is added to the light path, the combination of two polarizers will give underexposure to the picture. Many cameras including most slr auto focus cameras require the use of a circular polarizer. See your camera instruction manual to see if your camera model is a type that requires a circular polarizer.

My question is this...
Does my XTi use a beam splitting meter?
I have a polarized filter that I used at the beach, and the entire photo is stopped down. It did not produce very good colors, but that could be error on my part.

Yes. Use a circular polarizer with your XTi.

You lose between 1 and 2 stops of light when you use a polarizer, so bear that in mind. And they are also most effective at 90 degree angles to the sun (point the little white dot on the filter at the sun for max polarization). So if you are facing the sun, or if it's directly behind you, the polarizer will have little effect other than to lose you light and act like an ND filter.

The general rule is, if your camera has Auto-focus you need a circular polarizer, which are more expensive than 'linear' ones!