Spotting comment spam

Here at Snowulf, we get a LOT of comment spam.  On average we receive 48 spam comments a day (Thanks Akismet for the stats), most of these are to old articles and immediately dropped.  However, a few are to newer articles and we manually review them.  Most are fairly obviously spam, with links to hideous domain names or text that just makes no sense.  On rare occasion a spam message...