Due to limitations in Openboot PROMs, you can't boot any of the 32bit SPARCs (sun4c, sun4m, sun4d) from a root partition that has parts lying beyond the 2GB mark on a SCSI disk.
On systems with really old PROMs (revision 2.5 or less) you need to make the root partition smaller than 1GB.
The Ultra PROMs are capable of this, but Solaris prior to version 2.6 also has a bug which effectively prevents Ultras from booting from large root partitions too. Patch 103640-08 or later fixes this for Solaris 2.5.1, so later 2.5.1 HW releases should be OK too.
Typical error messages include:
bootblk: can't find the boot program boot: cannot find misc/krtld Short read. 0x2000 chars read Read error.