Would it be harmful to my rMBP if I installed Windows 10 when it is released? I was talking to someone about it, and because Apple uses non-standard hardware voltages could cause damage using the drivers for Windows 8.1
I am currently using Windows 10 TP (Technical Preview Build: 10240) on my MacBook Pro Late 2011. I am using Boot Camp with the drivers made for previous Windows versions. I haven't had any issues with using it but after upgrading to an newer build I was needing to reinstall the driver package again to get it to work properly.
Also, as Steve Chambers said in his answer, it won't physically break or damage your Mac. Only remember that if you are upgrading from a previous Windows version, make a backup in case you need to go back or do a clean install. In addition, it could be a good idea to have the Windows drivers ready on a USB flash drive just in case.
This is tested with the latest TP (Technical Preview) build of Windows 10 but could change in the final version (unlikely but could). If you are not in a hurry, wait a week and then do a Google search and see if anyone had any issues with using the released Windows 10 version on Boot Camp.
Rob has the gist of this because Apple has not released official Windows 10 drivers and installing Windows 10 on your Mac may or may not work.
That said often times you can use previous Windows version drivers on the next version. That often worked when Windows 7 came out, you could use Vista drivers and they usually worked.
So it is possible that installing Windows 10 by upgrading a working copy of Windows 8.1 would work. In fact we would all like to know if it did, or not. And what, if anything, did not work (EG sound, trackpad, video, etc.).
So you should be safe installing Windows 10 over top of 8.1. And it is unlikely that doing so will physically damage your Mac (YMMV) but you could very well make your current Windows installation unusable and have to reinstall.
I upgraded to Windows 10 on a Macbook Pro 2013 model. Everything work fine except hotkeys.
I reinstalled the apple keyboard driver and now those work!
Note: drivers can be found at https://support.apple.com/downloads/DL1720/en_US/BootCamp5.1.5621.zip
Upgraded to Windows 10 without any issues, ATI drivers will be replaced by standard Windows drivers and will limit you in extra options available for ATI Radeon R9, Sleep mode won't be available, also touch pad won't work properly as well.
Apple just released the Bootcamp 6 for Windows 10.
You can use Boot Camp to install Windows 10 on supported Mac models that have OS X Yosemite installed. System Requirements
You need support software (drivers) installed with Boot Camp to use Windows 10. This software is automatically downloaded when you use Boot Camp Assistant. Boot Camp supports 64-bit versions of Windows 10 when used with a supported Mac.
Windows 10 is available from Microsoft as an ISO file, an installation DVD, and a USB flash drive. You need an ISO file of the 64-bit version of the Windows 10 installer to install Windows on your Mac. If you don't have an ISO file, you can use Disk Utility to make one from your Windows installation DVD or USB flash drive. If you're installing Windows for the first time, make sure the Windows installer you're using is for a full installation (not an upgrade installer).