Smoothwall have made the following statement on the Smoothwall blog:
We can confirm that our version of OpenSSL is not vulnerable to this issue, and our version of GnuTLS has also been upgraded as of update73 to resolve another possible, but unrelated, SSL vulnerability, of which OpenSSL's is the latest of 3 recent issues in SSL implementations.
Smoothwall users are protected from Apple's recent bug (link below) by browsing through the web filter, however they are not immune to the "Heartbleed" issue where present on other web sites and services (though a MITM filtered connection is perhaps marginally harder to attack).
More information on each issue can be found here:
Apple "Goto fail"
The original blog post from Smoothwall is here: http://smoothwall.blogspot.com.au/2014_04_01_archive.html