Is it true that Windows 8 is writing a new bootloader (in the MBR) on every boot? Is it possible to have a Windows/Linux multiboot with GRUB (or BURG, I guess) as the bootloader? Or is this just complete speculator-y nonsense?
In a scenario where you have Ubuntu installed - dual booting with Windows 7, if you wish to upgrade Windows 7 to Windows 8, the GRUB remains intact and you can continue to use both Ubuntu and Windows. The GRUB lists "Windows 7 loader" and on selecting that Windows 8 boots successfully. There have been no issues so far.
However you may be careful when you choose to do a fresh installation. If you manage to destroy the GRUB, re-installing Ubuntu should do the trick. Next time you boot you would see no problems. So the rumors of Windows 8 writing new Boot loader is completely false.
Edit: Re-installing Ubuntu would not be necessary. Details in the comments.
You can use EasyBCD. It edits your Windows bootloader, and you don't need to worry about GRUB if you reinstall Windows (any version for that matter), as the Windows bootloader will always remain.