Evidence of automated comment spam on Moveable Type weblogs. It makes sense for spammers to concentrate on weblog software with significant market share, since they can then push out spam automatically. The infrastructure for relatively automated discovery of Moveable Type weblogs is already in place.
It becomes problematic when this automation reaches the point where it doesn't matter to the spammers whether an individual weblog is indexed by Google or not — then they come with the big watering can, just like with email today. And in the long run, there will probably be no way around registration requirements for comments (or at least some form of checking for humanity on the other end).
Things get really tricky when trackbacks are spammed — because these are inherently designed for automated ping distribution, moderating that becomes difficult. Sure, you can use heuristics to decide whether a spammer is a spammer, but they will occasionally fail.
Unlike wikis, weblogs are particularly interesting for web spammers due to their high prevalence and extensive interconnection — and many protocols (trackback, pingback, comments) are very much designed for openness.