OK, no one actually asked me this, but based on some recent press it seems that there may be confusion about what, exactly, dry hopping is and is not. To wit, "The pale ale is commonly pale golden or light copper with a moderate to strong hop aroma that comes from dry hopping, where hops are added late to the brew kettle. ... It's made with all-Cascade hops, which are added into the post-boil of the wort, with a light amount of dry hopping to give it enough bitterness for the style ...." The second sentence actually hurts me just below my spleen.
We talked about hops before. There we talked about alpha acids (the bittering component of hops) and beta acids (the aroma component) and essential oil (the aroma characteristics). It's a good review of how hops work; certainly not comprehensive, but it gives you a good overview. Friend of MBR, Joe Walts of RePublic Brewpub, talked quite extensively about hopping a few weeks ago. Joe looks at some hop-science, engages in some science himself, and brews up an IPA that was apparently pretty darn impressive.
But what's at issue with the dry hopping is how hops are used. Hops can be added throughout the beer-making process, from the lautering all the way up until it hits a bottle. We'll talk about some of those now (see, John Palmer, How to Brew):
First Wort Hopping: low alpha-acid hops are added pre-boil to the kettle as the wort moves from the lauter/mash tun to the kettle; "one study among professional brewers determined that the use of FWH [ed note: First Wort Hopping] resulted in a more refined hop aroma, a more uniform bitterness (i.e. no harsh tones), and a more harmonious beer overall compared to an identical beer produced without FWH" Because the hops being added are typical low in alpha acid to begin with, this addition does not add a significant amount of bittering and really just helps to add complexity to the hop profile.
Bittering: hops are boiled for usually 45-60 minutes, although many breweries (for example Dogfish Head) may boil for as long as 90 or even 120 minutes. During a bittering addition, the alpha acids of the hop are isomerized and the oils are boiled away leaving very little hop aroma.
You might be wondering why the essential oils of the hops added during the first wort hopping wouldn't also boil away. Well, according to Mr. Palmer: "by letting the hops steep in the wort prior to the boil, the oils have more time to oxidize to more soluble compounds and a greater percentage are retained during the boil."
Flavoring: the general trade-off with hops in the boil is that the longer the hops boil the more bittering they provide, while the shorter the time that the hops boil the more aroma is taken; flavoring additions occur mid-way through the boil and provide a good balance between aroma and bittering.
Aroma/Finishing: this includes hops added in the last 15 minutes of the boil and hops that are steeped post-boil; in this addition of hops, the essential oils are not lost to evaporation but the alpha-acids are not isomerized resulting in a very aromatic, but non-bitter hoppiness; in some cases a "hop back" is used where the wort is filtered through a bed of hops on its way to the chiller/filter/fermenter; in the case of hop backs and steepings, you can sometimes get tannin-y flavors from the leafy and stem materials from hops which are usually neutralized immediately in a boil
Dry Hopping: here, hops are added to the beer after the beer has been through a primary fermentation and it is sitting for a secondary or conditioning period. "If the hops are added to the fermenter while it is still actively bubbling, then a lot of the hop aroma will be carried away by the carbon dioxide." This imparts even more aroma and gives an opportunity to introduce greater complexity, no bitterness is imparted during this phase.
So "dry hopping" does not occur in the brew kettle post-boil and cannot give a beer "just enough bitterness for the style." It does provide awesome aromas though and probably does, in fact, account for some of the Cascade aroma of the US Pale Ale.
One last thing because it is a huge pet peeve of mine. A quote from Kirby Nelson, head brewer at Capital Brewery, regarding the US Pale Ale: "What I want to achieve with this beer is a nice example of the style." Which, perhaps explains why the Pale Ale is relatively unimpressive - a middle-of-the-road, style-guideline version of a pale ale is a really boring beer.