Marc,

Your question has been out here a few days, so I'll try to respond... but a little more information might be needed to get the exact response you're looking for.  I'll provide a few different possible answers...

The easiest way to decode an iso-8859 subject line is simply to deliver it to a mail client.  Both Outlook and Outlook Express feature automatic decoding.

Another way would be to search for a web page that provides this capability.  There are a few web pages around that provide online utilities that convert between different character sets and encoding methods.  You paste your encoded character string into a text area on the page, click a "convert" button and see the results.  Sorry, I don't have the address of any of these pages handy, but a Google search should produce one for you if this is what you're looking for.

To better answer your question, it would be helpful to know WHY you're wanting to do this.  Are you wanting to perform keyword matching on the decoded text?  If so, this might not be possible in SpamFilter as of yet... Roberto mentioned in this posting http://www.logsat.com/spamfilter/forums/showmessage.asp?messageID=1945 that the subject line is not currently being decoded.

A more direct approach might be possible for spam filtering of these messages, depending on your locality and the language(s) used in the email messages you process.  iso-8859 subject line encoding is NOT standard practice for text that can be represented in the Latin-1 character set.  On the other hand, this method of subject obfuscation is one of the spammer's favorite tricks.  Consider using a RegEx to quarantine ALL messages containing iso-8859 encoded subject lines.  This can be a very effective spam blocker in some small mail systems where all of the email communication occurs in languages that require only Latin-1 characters (English, French, Spanish, and similar languages).

You need to be careful when doing this, however, to ensure that you don't end up blocking legitimate messages.  If you run a mail system for a small company, and 100% of your email communication occurs within north America and/or western Europe, then you're probably safe in blocking all iso-8859 encoded subject lines.  Monitor your quarantine very closely for a few days, to ensure that you are not trapping legitimate messages.

If any of your users communicate in languages that require umlauts, dipthongs, different accents than customarily used in Spanish, French or English, etc... or if you're an ISP and must remain open to all languages... then this would not be a good idea for you.