An interesting article about tags, folksonomy, memes, social software and things like that on The Community Engine Blog: The difference between technorati and del.icio.us in spreading your meme
Despite initial appearances, Technorati’s approach to folksonomy is probably better oriented toward monitoring the spread of your meme (pivotal idea you are trying to promote). Del.icio.us is likely better oriented toward actually getting your meme adopted. The difference is in the number of resources to which you can apply your tag in either system.
Update: read also the great Salon article Steal this bookmark! about tagging.