Some owners have noted an issue with Apple's latest MacBook Pro model (early 2011), where the machine will start up to the gray Apple logo screen without a progress indicator and freeze. Apple has also noted the issue, releasing a knowledge base article to help with the situation.
Apple Support Discussions forum user CoryCripita writes:
"So for the second time my Macbook Pro is stuck on the ****** gray screen and won't boot. The first time this happened 2 days ago I HAD TO resort to Archive and install. The only thing that I can even think caused this was again the fact that I updated via Software Update then rebooted."
One suggestion offered on the forum was to reset the PRAM. To do so, turn off your machine. While holding Command + Option + P + R, turn your Mac back on. Wait for the boot chime to ring three times and your machine will start normally.
Apple has released a knowledgebase article to address some causes of this problem. According to the article, the Mac booting to the gray logo screen "may happen if the computer starts up from the hard drive or from an external drive using the wrong version or build of the operating system."
If this happens, reinstall the proper build of Mac OS X from your install DVDs that came with your Mac. Later versions of Mac OS X will also work, but the article notes that using a version of OS X that was released before the computer will likely cause this issue.
You can check and see which version of Mac OS X your machine is attempting to start from by booting your Mac in Verbose Mode. When starting up, hold Command + V. If a DOS-like white text on black screen pops up, you've done it correctly.
"If the computer has the wrong build of the OS installed, the startup process will stop. The following line of text will appear on the display: "Loading System\Library\Caches\com.apple.kext.caches\startup\Extensions.mkext.""
Hold the power button to shut down and quit Verbose Mode. When you next power on your Mac it will start normally.