More seriously, you might run into problems if the weather really turns nasty. Convertible pantsan example would be Columbia Sportswear's Trekker Convertible ($50; www.columbia.com)are almost always made of a light nylon material. This fabric is best suited for a warm-to-cool transition, or vice versa, not warm-to-cold or cool-to-cold. In other words, if it's 35 degrees, blowing, and raining, they won't do a lot for you.
Myself, I can't shake the geek look of long underwear worn under hiking shorts. I've found tights such as Patagonia's Silkweight Bottoms ($32; www.patagonia.com) to be comfortable across a surprisingly wide temperature range; warmth can be added very quickly by throwing on a pair of windproof or waterproof pants. Another good choice is a pair of pants using one of the new-generation fabrics like Schoeller Dryskin, such as L.L. Bean's Guide Pant ($90; www.llbean.com); I've used a pair and they're really terrific. Cloudveil's Prospector Pant ($90; www.cloudveil.com) is similardurable, wind and water resistant, and able to keep you comfortable from temps in the 60s down to freezing or below. These are one-piece pants, though.