Just for giggles, I looked up the definitions in Websters (sorry no Funk and Wagnals)

Plaid (n) [Gael. plaide, a blanket or plaid, said to be contr. from peallaid, a sheepskin, from pealle, a skin or hide]
1. a long piece of twilled woolen cloth with a checkered or crossbarred pattern, worn over the shoulders of Scottish highlanders.
2. cloth with a checkered or crossbarred pattern.
3. any pattern of this kind.

Tartan (n) [prob. merging of ME. tirtaine (from OFr. Tiretaine), mixed, fabric and ME. Tartarin, tarlayne (from OFr. tartarin, lit., cloth of Tartary), a rich material from China.
1. woolen cloth with a woven pattern of straight lines of different colors and widths crossing at right angles, worn especially in the Scottish Highlands, whereach each clan had its own pattern.
2. any plaid cloth like this.
3. any tartan pattern.
4. any article made of such a fabric. silk tartan; a silk textile having a tartan pattern.

Tartan (a) consisting of, made from, or resembling tartan; having the pattern of a tartan. tartan velvet; a velvet withe a short map and woven in tartan patterns

Oh and it is also a type of ship in Spanish and Italian.

Granted some of the info above is inaccurate, but it does explain whey people get confused.

Adam