[plt-scheme] finding the right version of libraries

I've got a scheme program that I want to eventually run as a shell 
script.  I'm using 299.100 and have the following at the top of the 
file:

#!/usr/local/plt/bin/mzscheme -r

(require (lib "smtp.ss" "net")
          (lib "head.ss" "net")
          (lib "xml.ss" "xml")
          (lib "match.ss")
          (lib "pregexp.ss"))

When I start mzscheme as /usr/local/plt/bin/mzscheme (the same command 
as above) and load this file with (load "..."), it runs perfectly, but 
when I run it as a script (i.e. just execute it from the shell prompt), 
I get

/Applications/PLT Scheme v208/collects/net/compiled/smtp.zo: read 
(compiled): code compiled for version 208, not 299.100

I think, I could solve this problem by uninstalling v208, but I've got 
two lectures left this term and would rather not chance the version 
change at this point.  Besides, I think there's something I need to 
learn here.

What's going on?

--phil--