From John Gruber's first impressions of iPhone: “Booting: A cold boot takes about 20 seconds. (Sleep/wake is effectively instantaneous – far faster than any Mac.)”
From Eastern Standard Tribe by Cory Doctorow:
There was a cheap Malaysian comm that he'd once bought because of its hyped up de-hibernate feature -- its ability to go from its deepest power-saving sleepmode to full waking glory without the customary thirty seconds of drive-churning housekeeping as it reestablished its network connection, verified its file system and memory, and pinged its buddy-list for state and presence info. This Malaysian comm, the Crackler, had the uncanny ability to go into suspended animation indefinitely, and yet throw your workspace back on its display in a hot instant.
When Art actually laid hands on it, after it meandered its way across the world by slow boat, corrupt GMT+8 Posts and Telegraphs authorities, over-engineered courier services and Revenue Canada's Customs agents, he was enchanted by this feature. He could put the device into deep sleep, close it up, and pop its cover open and poof! there were his windows. It took him three days and an interesting crash to notice that even though he was seeing his workspace, he wasn't able to interact with it for thirty seconds. The auspicious crash revealed the presence of a screenshot of his pre-hibernation workspace on the drive, and he realized that the machine was tricking him, displaying the screenshot -- the illusion of wakefulness -- when he woke it up, relying on the illusion to endure while it performed its housekeeping tasks in the background. A little stopwatch work proved that this chicanery actually added three seconds to the overall wake-time, and taught him his first important user-experience lesson: perception of functionality trumps the actual function.
All week I wondered about iPhone's responsiveness after "putting it to sleep" (no, as a good Canadian, I have to wait to get one), and John says it's zippy. I still can't help but wonder, though, if Apple pulled one over on people like the Malaysians did on Art.