Hello,
When macroexpanding to a buffer, I had the same problem as the one described in the following message (I use Emacs 21.3.1):
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/94879
The workaround proposed there seems to do the trick. (The function to modify is `slime-eval-macroexpand'.)
Ariel