Update, 11/10/2008: These tests have been superseded by a new set of updated tests which use the latest Vista service pack and a single hard drive. Microsoft says that "a very good system boots in under 15 seconds," and while the early-release Windows 7 Preview doesn't deliver a sub-15 second boot time, it does start 20% faster than Windows Vista on my computer. The only way to truly compare boot-up times between operating systems is to try them out on the same exact hardware, and as of today I'm triple-booting XP, Vista, and the Windows 7 Preview on a newly-built PC. The results? Windows 7 boots much faster than Windows Vista—but surprisingly, no faster than Windows XP. Here are my exact numbers, machine specs, and caveats.
Update, 11/10/2008: These tests have been superseded by a new set of updated tests which use the latest Vista service pack and a single hard drive. The hardware: My test system has an Intel Core 2 Duo processor running at 3.16GHz with 4GB of RAM. The only difference between XP, Vista, and the Windows 7 Preview is that Win7 is installed on an IDE hard drive, and XP and Vista are booting from SATA drives. That puts Win7 at a slight disadvantage because IDE drives are not as performant as SATA. Also: the version of Windows 7 that I'm using is the Preview release from last month's PDC conference, an incomplete, earlier-than-beta, for-testers-only edition. One can only assume it will get better (read:faster) in the final release. The test: Since I'm triple-booting the three operating systems, I used a simple handheld timer to get my numbers, starting at the "Choose your operating system" screen (which comes after the BIOS startup) and ending at the user login screen. These tests do not include the amount of time it takes to load the user's desktop, because that depends entirely on what's in your Windows startup. This is simple, pure, operating system boot here—no desktop or BIOS involved. 
Here are my final numbers from starting the three operating systems on the same box and timing them:
Update, 11/10/2008: These tests have been superseded by a new set of updated tests which use the latest Vista service pack and a single hard drive. Final result: The Windows 7 Preview boots 20.6% faster than Windows Vista, but in exactly the same amount of time as Windows XP. Because of the slower IDE drive on my test system, and the preview status of Windows 7, it's very probable that Win7 will be even faster than XP in its final releases. On a related note, only 23% of you said your computer boots up in under 30 seconds, so it sounds like "faster boot time" might be one of the less sexy but much-appreciated features coming in Windows 7.
Here are my final numbers from starting the three operating systems on the same box and timing them:
| Operating System | Time to reach user login prompt |
|---|---|
| Windows 7 Ultimate (Preview, PDC edition) | 27 seconds |
| Windows Vista Ultimate Update: Without Service Pack 1 installed! | 34 seconds |
| Windows XP Professional | 27 seconds |