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 */
