After some poking around, the following is enough to reproduce:

On Thu, Sep 03, 2009 at 02:27:28AM +0900, Czarek wrote:
> Sorry for not submitting a patch, but I'm probably too dumb to work
> out the cause.
> 
> Problem:
> ========
> Got latest ruby, fired up rails app, segfault - every time, same place
> 

Actually, the following is enough:

  ---------- a.rb ------------
  a = 'a'
  "#{a} " "#{a} " "value #{a}"
  ----------------------------

---------- GDB session -----------------
$ gdb --args ruby ./a.rb
 ..... (snip) .......
(gdb) run
Starting program: /usr/local/bin/ruby ./a.rb
[Thread debugging using libthread_db enabled]
[New Thread 0x7ffff7ff1910 (LWP 26329)]

Program received signal SIGSEGV, Segmentation fault.
0x000000000046a099 in list_concat_gen (parser=0x92f740, head=0x81b370, tail=0x81b1e0) at parse.y:7785
7785	    last->nd_next = tail;
-----------------------------------------

Can anyone confirm this?  

-- 
Cezary Bagi• ß‘ki