<hijack>Why? I ask because when I chilled my last brew I bulk primed and bottled it at the cooled temp, which I'm guessing was a not-so-cool 8 degrees.</hijack>
Once the brew is primed and in the bottle, the yeast needs to ferment out the priming sugar in order for it to carbonate. Some yeasts will do that at 8C, but it will take quite some time. Some yeasts will just flocculate out into the bottom of the bottle at this temperature, leaving you with flat beer.
So you ferment, chill, prime and bottle, then raise back to fermentation temperature for the yeast to work. After 2 weeks or so (maybe less, depending on how much fermentable sugar the yeast needs to work through to carbonate), you then do one of three things...continue to store at cool-ish ambient, store at cellar temp (8-12C), or store cold.
edit: I was referring to the bottles themselves being at fermentation temperature whilst the yeast works. You don't need to raise it back up
before it goes into the bottle, if the prime was calculated taking the maximum fermentation temp into consideration (like you did, from memory). I usually pull straight out of cold conditioning, prime it, bottle it, then just let the bottles warm up to 20. Leave it at that for 10-14days, then put in a cool spot in the house.