Code:
-------------------------------------------------------------
package main

type MyInterface interface {
}

type My struct {
    a, b, c int
}

var my = &My{1,2,3}
var myInterface MyInterface = my
var origMy, ok = myInterface.(*My)

func main() {
}
-------------------------------------------------------------

Compilation gives:
-------------------------------------------------------------
[nsf @ waytogo]$ 8g test.go
defn [9f9cbc0]
.   AS2DOTTYPE l(12) x(-1000000000) tc(1)
.   AS2DOTTYPE-list
.   .   NAME-origMy G0 u(1) a(1) l(12) class(1) tc(1) *My
.   .   NAME-ok G0 u(1) a(1) l(12) class(1) tc(1) bool
.   AS2DOTTYPE-rlist
.   .   DOTTYPE2 l(12) x(-1000000000) tc(1) *My
.   .   .   NAME-myInterface G0 u(1) a(1) l(11) class(1) tc(1) MyInterface
test.go:15: internal compiler error: init1: bad defn
-------------------------------------------------------------

Compiler/machine spec:
linux, 386, b761e0299e9b release/release.2010-07-01

Even if it's not a legal declaration, good error message is expected.