There are going to be a bunch of answers, Sierra Nevada use a Torpedo, basically a hop back during fermentation and say it gives a different result to just dry hopping the beer like a lot of other brewers do.
Which is I suppose is sort of where I think the answer will lie. Hot extraction will give different flavours and aromas to a cold extraction, just try making Iced Tea, cooled down tea looks, tastes and smells a lot different to tea leaves soaked in cold water in the fridge overnight yet both are just tea and water.
With a hot wort, well we all know hot dissolves things faster and more effectively than does cold, it will also dissolve different things, the wort sugars will block some flavours and increase extraction of others, alcohol is a solvent for some chemicals that wont readily extract in wort (hot or cold).
Pellets are just hop cones ground up real fine and extruded through a die, problem is they are real fine and will go through anything but a very fine filter and a very fine filter will block up in next to no time, if you T-Bagged some hops, put the T-Bag in a hop back and ran hot (or cold) wort through and around them on the way from the kettle to the fermenter that might give you a reasonable extraction without all the hops just washing over into the fermenter (sic dry hoping the fermenter)
Personally I don't think you will get anything like as much hop character as you would like with a cold extraction, it is a good way to get more aroma, aroma molecules are by definition volatile and although the extract well into hot wort they are also ejected quickly, which is one reason a hop back is traditionally between the kettle and the chiller.
Mark
