For people without older BeOS-compatible hardware it may be easier to run Haiku in a virtual machine, for now. It’s possible to install BeOS(+patches) or prepare a Haiku partition on older, supported hardware, and then move the disk to a more recent computer. Haiku should be fine on its own (without anything BeOS) on a lot of modern hardware, but when installing Haiku, preparing partitions etc, we still rely somewhat on having BeOS (or Zeta) simply because, most of us still do.
(If anyone spots anything wrong here, please do point it out!)
A BeOS+Haiku user would be able to just wipe a BeOS partition and install Haiku in its place, leaving the BeOS bootcode in the partition boot record, for GRUB or some other MBR-installed boot manager to chainload. Starting from Linux, I’m not sure exactly how to actually install Haiku to a partition, having built it. I skimmed the documentation but didn’t see anything truly useful, except caution that it doesn’t work as expected. 
Here’s one way to do steps 1-3 mentioned above:
If you have a BeOS CD, a suitable harddisk on PATA (SATA not supported by BeOS) and your hardware isn’t somehow too new for BeOS, this should be possible:
Boot from a BeOS (or maybe Zeta) CD. In the Installer, press “More options”, or the triangle. Press the button “Setup Partitions…”. This Opens DriveSetup.
In DriveSetup, select the physical disk where you have a spare partition for Haiku, then choose Setup-Partition-Intel, and proceed. In the partition window that opens, be extra careful not to touch any partitions you wish to keep. Do only this: for the Haiku partition, set the partition type to BeOS, and press OK. Back in the main window with the list of disks, select the physical disk, then choose Setup-Initialize-(your Haiku partition)-Be File System. Initialize does the same as quick format in Windows. Be careful to select the right partition! In the initialize options window, select 4096 as block size and name it Haiku or something.
Close DriveSetup and return to the Installer.
You need to make the partition bootable, by adding the BeOS bootcode to the very start of the Haiku partition. This is done by the command-line utility ‘makebootable’ which is run from within BeOS, like this, “makebootable /MyHaikuVolume” (note: the mount point, not the device node). To do this from the BeOS install CD you need to open a terminal. You do this by pressing Ctrl-Alt-Shift-T, in the Installer. (This is a service entry of sorts.) Makebootable only works on mounted volumes, IIRC, so you need to mount it if it’s not already. You can check with ‘df’ and you can double-check with DriveSetup. Then type “makebootable /MyHaikuVolume” (or whatever you named it).
The partition should be ready for Haiku now, and unless Linux, GRUB or the Haiku build in Linux somehow overwrites the partition boot record you should be able to boot into Haiku, by chainloading the Haiku partition from GRUB, if the Haiku files are all in place and the filesystem is proper BFS.
I’m not sure how well the Haiku build system supports installing to a partition from within Linux. You may have to ask in IRC channel #haiku on freenode or search the website. I know the development team are very much interested in making Haiku development more accessible to non-BeOS people, as the team needs more people, and most active BeOS-developers are already involved.
