I have a SunFire 280R that I've been experimenting with, and decided to try Debian on it.
The release notes for 4.0 say:
3.2 Issues with booting on SPARC
It has been reported by several users that the installation CD fails to boot successfully upon the 'boot cdrom' PROM command, displaying the error 'Illegal Instruction'.
The apparent explanation for this problem is that it doesn't work because the machine had previously been rebooted from Solaris. The workaround is to power the machine off fully, and then boot it directly into the installation CD.
The problem was reported by users of various systems (namely, Enterprise 450, Blade 2000, Fire V240, Enterprise 250, Blade 100 and Enterprise 220R at the time of writing), so it is believed to be generic. Please let us know if you observe similar issues with your hardware.
It definitely is an issue on a 280R, but it's not because "the machine had previously been rebooted from Solaris". I had OpenBSD on this machine previously, and I still got the illegal instruction issue. And ran into the "qla2xxx is non-free" issue too. Very annoying, almost makes me wish I'd stuck with Solaris on this box, but sunfreeware isn't quite up to snuff for my purposes (harder to ensure you're running bug-free versions of things) and OpenBSD is a PITA to do mirroring of root partitions.