bug or feature? Ctrl+C sometimes can't interrupt a loop with multiple enclosed pipes

[Ilya Basin]

Configuration Information [Automatically generated, do not change]:
Machine: i686
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i686' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i686-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -march=i686 -mtune=generic -O2 
-pipe
uname output: Linux IL 2.6.33-ARCH #1 SMP PREEMPT Thu May 13 12:06:25 CEST 2010 
i686 Intel(R) Core(TM)2 Duo CPU E6550 @ 2.33GHz GenuineIntel GNU/Linux
Machine Type: i686-pc-linux-gnu

Bash Version: 4.1
Patch Level: 7
Release Status: release

Description:
Sample script included.
When I press Ctrl+C, only the inside loop is interrupted. The outside loop 
keeps going.
In zsh and in old plain sh Ctrl+C works.

Repeat-By:
run this sample script and try to interrupt it with Ctrl+C:
#########################
  { echo 1; echo 2; echo 3; } | while read a; do
    echo $a
    { echo 1; echo 2; echo 3; } | while read b; do
      echo $a-$b
      sleep 1
    done
  done
#########################