Steinar, you made my day!
Here's a fixed version::
#! /bin/sh
WORK="`mktemp`"
wget -O- "http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=$1&mbox=yes" > "$WORK"
mutt -f "$WORK" # -e 'push D~=\n$'
rm "$WORK"
Now, it would be nice if it integrated with bts cache...
abi on #debian-devel suggests to use::
bts show --mbox <bugid>
Wow! I didn't know it existed! And indeed it integrates with bts cache :)