Apply by doing:
cd /usr/src
patch -p0 < 023_altivec.patch
Then build and install a new kernel.
Index: sys/arch/powerpc/powerpc/trap.c
===================================================================
RCS file: /cvs/src/sys/arch/powerpc/powerpc/trap.c,v
retrieving revision 1.72
retrieving revision 1.72.2.1
diff -u -r1.72 -r1.72.2.1
--- sys/arch/powerpc/powerpc/trap.c 2005/12/17 07:31:27 1.72
+++ sys/arch/powerpc/powerpc/trap.c 2007/04/26 23:56:01 1.72.2.1
@@ -1,4 +1,4 @@
-/* $OpenBSD: trap.c,v 1.72 2005/12/17 07:31:27 miod Exp $ */
+/* $OpenBSD: trap.c,v 1.72.2.1 2007/04/26 23:56:01 ckuethe Exp $ */
/* $NetBSD: trap.c,v 1.3 1996/10/13 03:31:37 christos Exp $ */
/*
@@ -645,6 +645,12 @@
trapsignal(p, SIGILL, 0, ILL_ILLOPC, sv);
break;
#endif
+
+ case EXC_VECAST|EXC_USER:
+ KERNEL_PROC_LOCK(p);
+ trapsignal(p, SIGFPE, 0, FPE_FLTRES, sv);
+ KERNEL_PROC_UNLOCK(p);
+ break;
case EXC_AST|EXC_USER:
uvmexp.softs++;
Index: sys/arch/powerpc/include/trap.h
===================================================================
RCS file: /cvs/src/sys/arch/powerpc/include/trap.h,v
retrieving revision 1.6
retrieving revision 1.6.12.1
diff -u -r1.6 -r1.6.12.1
--- sys/arch/powerpc/include/trap.h 2003/04/26 08:01:42 1.6
+++ sys/arch/powerpc/include/trap.h 2007/04/26 23:56:01 1.6.12.1
@@ -1,4 +1,4 @@
-/* $OpenBSD: trap.h,v 1.6 2003/04/26 08:01:42 jmc Exp $ */
+/* $OpenBSD: trap.h,v 1.6.12.1 2007/04/26 23:56:01 ckuethe Exp $ */
/* $NetBSD: trap.h,v 1.1 1996/09/30 16:34:35 ws Exp $ */
/*
@@ -51,6 +51,7 @@
#define EXC_VEC 0x0f20 /* AltiVec Unavailable */
#define EXC_BPT 0x1300 /* Instruction Breakpoint */
#define EXC_SMI 0x1400 /* System Management Interrupt */
+#define EXC_VECAST 0x1600 /* AltiVec Assist */
/* And these are only on the 603: */
#define EXC_IMISS 0x1000 /* Instruction translation miss */
