What steps will reproduce the problem?
1. go run this program:

package main

type Big [50]int64

func main() {
    seen := make(map[Big]bool)
    pow := Big{0: 2}
    for b := 0; b < 100; b++ {
        seen[pow] = true
    }
}

What is the expected output? What do you see instead?

Expected: nothing.
Got:
$ go run crash.go
panic: invalid memory address or nil pointer dereference
throw: panic during gc
[signal 0xb code=0x1 addr=0x0 pc=0x404ada]

goroutine 1 [running]:
main.main()
    /tmp/crash.go:9 +0xa6

goroutine 2 [syscall]:
created by runtime.main
    /build/src/release-build/src/pkg/runtime/proc.c:221
exit status 2

GDB trace:
(gdb) run
Starting program: /tmp/crash 
[New LWP 15165]

Program received signal SIGSEGV, Segmentation fault.
[Switching to LWP 15165]
ReleaseN (c=void, l=void, n=void, sizeclass=void) at
/opt/remy/go/src/pkg/runtime/mcache.c:63
63          lp = &(*lp)->next;
(gdb) bt
#0  ReleaseN (c=void, l=void, n=void, sizeclass=void) at
/opt/remy/go/src/pkg/runtime/mcache.c:63
#1  0x0000000000404cdb in runtime.MCache_ReleaseAll (c=void) at
/opt/remy/go/src/pkg/runtime/mcache.c:126
#2  0x0000000000407465 in stealcache () at /opt/remy/go/src/pkg/runtime/mgc0.c:882
#3  0x0000000000407869 in runtime.gc (force=void) at
/opt/remy/go/src/pkg/runtime/mgc0.c:998
#4  0x000000000040fea3 in runtime.mallocgc (size=void, flag=void, dogc=void,
zeroed=void) at /opt/remy/go/src/pkg/runtime/malloc.goc:97
#5  0x0000000000410aa5 in runtime.mal (n=void) at
/opt/remy/go/src/pkg/runtime/malloc.goc:432
#6  0x000000000040244f in hash_subtable_new (h=void, power=void, used=void) at
/opt/remy/go/src/pkg/runtime/hashmap.c:73
#7  0x0000000000402dad in hash_grow (t=void, h=void, pst=void, flags=void) at
/opt/remy/go/src/pkg/runtime/hashmap.c:240

I use linux/amd64.
Happens with go1 as well as tip (go version weekly.2012-03-27 +d9a1980f5814)