I just tried the 1.5.1 beta on my quadra660av. I did a full install 
with the installer (1.1h) after reformatting the external disk drive. 
The boot fails with a panic: no init. I looked using the mini-shell 
in the installer and there is an init located at /sbin/init. The full 
directory parameters are:
-r-xr-xr-x 1 0 237568 Apr 13 11:48 init

The 1.5.1 alpha kernel that Hauke Fath posted also failed with a no init panic.

The output from the 1.5.1Beta boot follows:

...
NetBSD 1.5.1_BETA (GENERICS) #1: Fri Apr 13 20:33:29 EDT 2001
	briggs@oldpuma.macbsd.com:/b/src/sys/arch/mac68k/compile/GENERIC
Apple Macintosh Centris 660AV (68040)
cpu: delay factor 800
total memory = 69632 KB
avail memory = 60528 KB

...

PRAM: 0x3adc4bd3, macos_boottime:0x3adc4bc8.
root file system type: ffs
exec /sbin/init: error 8
init: not found
panic: no init
Stopped in init at 	_cpu_Debugger+0x6:	unlk a6
db>trace
_cpu_Debugger(ffffcfe8,ffffcff1,0,161ff8,87bfa8) + 6
_panic(2b8d7,2b8c6,2c934,0,0) + 60
_start_init(87c000) + 1ce
_proc_trampoline() + 2
db>
-- 
Tom Jernigan,  jernigantc@ornl.gov
presently at General Atomics in San Diego, Bldg 13/551
Phone (858) 455-4122, FAX (858)455-4515 NOTE NEW AREA CODES