I’ve gotten the last two OpenSolaris source releases (20050701 and 20050720) to build without a problem on my Ultra 60. Build time went from 4:00 (initial OS release, building on top of Solaris Express) to almost 6:30 (building OS on top of itself, in debug mode) but I had no problems. Looks like they’ve updated the BFU install process to automatically run acr to resolve conflicts.
Sun Microsystems Inc. SunOS 5.11 mrbill Jul. 27, 2005 SunOS Internal Development: root 2005-07-27 [mrbill] bfu'ed from /opensolaris/mrbill/archives/sparc/nightly on 2005-07-28 Sun Microsystems Inc. SunOS 5.11 snv_16 October 2007