Mark Sirota , Sep 24, 2007; 10:37 p.m.
The quick and dirty way -- Select the shots you want to crop in Grid, and then in the Quick Develop panel on the right, pull down the Crop Ratio list and select 2x3.
If you want to be a little more careful about your crops, then you need to do it one shot at a time. Enter the crop mode with the R shortcut, then pull down the "Aspect" list in the Toolbar and choose 2x3. Then adjust the crop rectangle to taste.
Yeah, it's weird that's called "Crop Ratio" in one place and "Aspect" in another, and it's weird that both 2x3 and 4x6 are in the list.