From Cellar Notes

“Wine is served in stemware because the temperature at which wine is served can have a profound impact on the tastes and the enjoyment it yields. Wine glasses should always be held by the stem of the glass rather than the bowl since the heat of your hand will quickly warm the liquid.
Warming a wine above its desired serving temperature will yield unwanted and unpleasant characteristics. The alcohol in wine will give a sharpness or ‘bite’ to the taste if the wine is served above 74 F. degrees.”

From eHow.com

To avoid warming a wine above its suggested temperature, hold the wine by the stem.
Pinch the stem of the glass between your index finger and thumb.
When drinking do not place your palm on the base of the glass.

From Milos Pesic for EZine Articles on Wine Glasses

Used for drinking and tasting wine, the proper use of wine glasses is also an important part of cuisine etiquette. Part of the frequently neglected yet highly obvious thing about the correct use of wine glasses is the stem part of the glass. The proper manner to drink a wine from wine glasses is to hold its stem, then drink. It is not just for social or elegance purposes, yet scientifically, holding the wine glass this way matters the wine temperature.

The temperature of the wine, which turns out to play a favorable factor in the overall taste of the drink, is not at all affected if your follow the correct way of holding wine glasses. This is obtained since the stem of the glass is not directly in contact with the drink. The wine temperature will be a lot harder to control had one grasped the glass by its bowl since it is directly in contact with the drink.

From Bella Online
Holding the Wine Glass:

“Only hold the glass by the stem. The heat from your hand may heat up the wine and change the flavor of the wine. Rumour has it that you should hold red wine glasses by the bowl. No. The serving temperature of red wine is in the low 60sF. The heat of your hand will warm it too much. The only exceptions to the “no hands” rule are when drinking Cognac or brandy. Warming is desirable in these situations.”

Video on How to Hold a Wine Glass