Those big christmas light bulbs burn pretty hot, it would'nt take many to heat up a coldframe, depending on the size of the frame, maybe one... If you use an intense source of heat it's hard to get an even distribution...
(just thinking outloud...)

I've seen a couple different 'heaters' in these forums that may be more economical/effective.
Both involve the use of water to hold/distribute heat from either a standard lightbulb suspended over water in a styrofoam frame, or a fishtank heater in a sealed water vessel located inside an insulated 'box'.

Consider wattage and the amount of air space you want to heat... it seems to me that there are options more efficient than those old strings of big bulbs.

I use the small twinkle type lights to keep my 10 gallon aquarium from freezing. I turn a child's swimming pool over on the tank, and tuck a long string of the lights, all balled up, under the pool with the tank. They are warm to the hand, but do not burn. It doesn't get too cold here, but it got to 29 twice so far without freezing.

I used a partial string of the old lights to warm my cold frame when temps dipped to minus 13 Celcius early Jan. Very unsual for zone 8. All my plants survived! I unscrewed a couple of bulbs. My bulbs are cool enough for me to grab and hold them with my hand.

Here's what I did

http://f1.pg.photos.yahoo.com/ph/byron101540/album?.dir=/75aa&.src=ph&store=&prodid=&.done=http%3a//f1.pg.photos.yahoo.com/ph/byron101540/my_photos

http://f1.pg.photos.yahoo.com/ph/byron101540/detail?.dir=/Greenhouse&.dnm=Boxed+in+area.jpg

The big what if is insulation to help keep the plants warm enough

i asked this same question in the green house forum and everyone told me i would just be heating the outside air of my green house. I didn't really understand how that made any sense but i forgot all about it after that,

Plenty of milk jugs with water and the lights maybe either wrapped around them or at least suspended on the floor (maybe stapled to a plank) might work. The thermal mass of the water in the milk jugs might moderate the temperature swings and lower position of the lights helps circulate the air (hot air rises.) Good luck.