Using a March 1 kernel I got several of these after adding a second hard 
disk:

esp0: timed out [ecb 0xf85d8948 (flags 0x12, dleft 8000, stat 0)], 
<state 3, nexus f85d88f0, phase(c 0, p 101), resid 8000, msg(q 0,o 0) > AGAIN 

sd0(esp0:0:0): esp0: timed out [ecb 0xf85d88f0 (flags 0x2, dleft 8000, stat 0)],
<state 3, nexus f85d88c4, phase(c 0, p 101), resid 2000, msg(q 0,o 0) >

In fact, they kept repeating until I rebooted the computer. The dmesg 
output is included at the end.

I hope this makes more sense to someone than is does to me.

Peter

------ dmesg output ------

esp0: timed out [ecb 0xf85d8974 (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d891c, phase(c 0, p 101), resid 8000, msg(q 0,o 0) > AGAIN
sd0(esp0:0:0): esp0: timed out [ecb 0xf85d891c (flags 0x2, dleft 8000, stat 0)], <state 3, nexus f85d8898, phase(c 0, p 101), resid 2000, msg(q 0,o 0) >
esp0: timed out [ecb 0xf85d891c (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d8898, phase(c 0, p 101), resid 2000, msg(q 0,o 0) > AGAIN
sd0(esp0:0:0): esp0: timed out [ecb 0xf85d89a0 (flags 0x2, dleft 8000, stat 0)], <state 3, nexus f85d8948, phase(c 0, p 101), resid 8000, msg(q 0,o 0) >
esp0: timed out [ecb 0xf85d89a0 (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d8948, phase(c 0, p 101), resid 8000, msg(q 0,o 0) > AGAIN
sd0(esp0:0:0): esp0: timed out [ecb 0xf85d8948 (flags 0x2, dleft 8000, stat 0)], <state 3, nexus f85d88f0, phase(c 0, p 101), resid 8000, msg(q 0,o 0) >
esp0: timed out [ecb 0xf85d8948 (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d88f0, phase(c 0, p 101), resid 8000, msg(q 0,o 0) > AGAIN
sd0(esp0:0:0): esp0: timed out [ecb 0xf85d88f0 (flags 0x2, dleft 8000, stat 0)], <state 3, nexus f85d88c4, phase(c 0, p 101), resid 2000, msg(q 0,o 0) >
esp0: timed out [ecb 0xf85d88f0 (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d88c4, phase(c 0, p 101), resid 2000, msg(q 0,o 0) > AGAIN
sd0(esp0:0:0): esp0: timed out [ecb 0xf85d89cc (flags 0x2, dleft 8000, stat 0)], <state 3, nexus f85d8974, phase(c 0, p 101), resid 8000, msg(q 0,o 0) >
esp0: timed out [ecb 0xf85d89cc (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d8974, phase(c 0, p 101), resid 8000, msg(q 0,o 0) > AGAIN
sd0(esp0:0:0): esp0: timed out [ecb 0xf85d8974 (flags 0x2, dleft 8000, stat 0)], <state 3, nexus f85d891c, phase(c 0, p 101), resid 8000, msg(q 0,o 0) >
esp0: timed out [ecb 0xf85d8974 (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d891c, phase(c 0, p 101), resid 8000, msg(q 0,o 0) > AGAIN
sd0(esp0:0:0): esp0: timed out [ecb 0xf85d891c (flags 0x2, dleft 8000, stat 0)], <state 3, nexus f85d8898, phase(c 0, p 101), resid 2000, msg(q 0,o 0) >
esp0: timed out [ecb 0xf85d891c (flags 0x12, dleft 8000, stat 0)], <state 3, nexus f85d8898, phase(c 0, p 101), resid 2000, msg(q 0,o 0) > AGAIN
device not ready
rebooting

Copyright (c) 1982, 1986, 1989, 1991, 1993
	The Regents of the University of California.  All rights reserved.

NetBSD 1.1A (PIAU.SCSI) #3: Thu Feb 29 03:05:06 MET 1996
    petersv@piau.df.lth.se:/usr/src/sys/arch/sparc/compile/PIAU.SCSI
real mem = 16723968
avail mem = 14184448
using 204 buffers containing 835584 bytes of memory
bootpath: /sbus0/esp0/sd@0,0
mainbus0 (root)
cpu0 at mainbus0: Sun 4/60 (MB86900/1A or L64801 @ 20 MHz, WTL3170/2 FPU)
cpu0: 65536 byte write-through, 16 bytes/line, sw flush cache enabled
memreg0 at mainbus0 ioaddr 0xf4000000
clock0 at mainbus0 ioaddr 0xf2000000: mk48t02 (eeprom)
timer0 at mainbus0 ioaddr 0xf3000000
auxreg0 at mainbus0 ioaddr 0xf7400000
zs0 at mainbus0 ioaddr 0xf1000000 pri 12, softpri 6
zs1 at mainbus0 ioaddr 0xf0000000 pri 12, softpri 6
fdc0 at mainbus0 ioaddr 0xf7200000 pri 11, softpri 4: chip 82072
audio0 at mainbus0 ioaddr 0xf7201000 pri 13, softpri 4
sbus0 at mainbus0 ioaddr 0xf8000000: clock = 25 MHz
dma0 at sbus0 slot 0 offset 0x400000: rev 1
esp0 at sbus0 slot 0 offset 0x800000 pri 3: ESP100 25Mhz, target 7
scsibus0 at esp0
esp0 targ 0 lun 0: <EMULEX, MD21/S2     ESDI, A00> SCSI0 0/direct fixed
sd0 at scsibus0: 312MB, 1224 cyl, 15 head, 34 sec, 512 bytes/sec
esp0 targ 0 lun 1: <EMULEX, MD21/S2     ESDI, A00> SCSI0 0/direct fixed
sd1 at scsibus0: drive offline
esp0 targ 1 lun 0: <QUANTUM, ELS42S, 3.06> SCSI2 0/direct fixed
sd2 at scsibus0: 40MB, 1536 cyl, 1 head, 53 sec, 512 bytes/sec
esp0 targ 2 lun 0: <CDC, 94171-9, 0045> SCSI1 0/direct fixed
sd3 at scsibus0: 312MB, 1549 cyl, 9 head, 45 sec, 512 bytes/sec
le0 at sbus0 slot 0 offset 0xc00000 pri 5: address 08:00:20:08:5d:3d
le0: 8 receive buffers, 2 transmit buffers
bwtwo0 at sbus0 slot 3 offset 0x0: SUNW,501-1419, 1152 x 900 (console)
bwtwo0: attached to /dev/fb