Recently, I'm getting similar panics on my alpha:
 NetBSD 1.4Y (VEGA) #31: Fri May 12 16:56:01 JST 2000
    iwamoto@vega.sat.t.u-tokyo.ac.jp:/usr/src/syssrc/sys/arch/alpha/compile/VEGA

I experienced no panics when running cvs update (with a NFS mounted
repository). All panics were during running /etc/daily, IIRC.
I suspect memory leaks because it takes 7-10 days before panics.
I will disable softdeps and see if a panic happens again.

db> tr
cpu_Debugger() at cpu_Debugger+0x4
panic() at panic+0xe4
lockmgr() at lockmgr+0x6d8
genfs_lock() at genfs_lock+0x28
vn_lock() at vn_lock+0x64
vget() at vget+0x124
ufs_ihashget() at ufs_ihashget+0xa4
ffs_vget() at ffs_vget+0x64
flush_pagedep_deps() at flush_pagedep_deps+0x150
softdep_sync_metadata() at softdep_sync_metadata+0x2c8
ffs_fsync() at ffs_fsync+0x288
vinvalbuf() at vinvalbuf+0x108
vclean() at vclean+0x110
vgonel() at vgonel+0x60
getnewvnode() at getnewvnode+0x240
ffs_vget() at ffs_vget+0xb4
ffs_valloc() at ffs_valloc+0xf0
ufs_mkdir() at ufs_mkdir+0xcc
sys_mkdir() at sys_mkdir+0x1bc
syscall() at syscall+0x1dc
XentSys() at XentSys+0x50
--- syscall (136, netbsd.sys_mkdir) ---
--- user mode ---

--
IWAMOTO Toshihiro