Under iOS 4, if you're using an iPhone 4 or iPhone 3GS, when you double click the Home Button the UI slides up and reveals the multitasking fast app switcher "behind" the dock. That certainly fixes the problem of moving between apps quickly, but it sacrifices everything the double click did on previous iOS versions.
As recently as a few weeks ago under iOS 3 (iPhone 3.0), double clicking the Home Button would send you to Phone Favorites. Under Settings, you could change that to Spotlight Search, Camera, or iPod Controls. (iPod controls were, and remain, the default behavior of a Home Screen double click if your iPhone is locked and audio is playing). Under iOS 4, those settings are gone.
Briefly under one of the iOS 4 betas, double click would launch the fast app switcher and double click and hold would launch phone favorites. This seems like a great alternative but it disappeared in later betas and never returned. (See our iOS 4 walkthrough for more.)
Sure, with iOS 4 multitasking you can use the fast app switcher to quickly get to the Phone App, and saved state means that if it was last on Favorites, it will return you right back to favorites. There are also widgets just to the left, which are slightly more complicated to get to but do take the place of popup iPod Controls. So, in some ways, the loss of old functionality is mitigated somewhat by the addition of new functionality -- but not the loss of elegance.
Perhaps the settings could return but instead of choosing what launches on double click, there could be a way to pin Phone Favorites, Spotlight, or Camera to the first spot in fast app switcher when it's launched. It would be one additional action -- click, click, tap instead of just click, click -- but it would be usable. Better yet, triple click can currently be assigned to bringing up the accessibility options. If you turn that off, how about letting us re-assign the triple click to the old double click options?
In the meantime, let us know what were you using double click for under iOS 3, if you miss it under iOS 4, and if you've found any good work arounds for it? Or is the new fast app switcher and widgets enough?