I always liked the idea of deleting tweets. It can play havoc with Twitter's systems which are not designed for bulk-deletion of tweets. I deleted 10000 tweets once, and was left in limbo for months as the twitter DB was refreshed to accommodate such a massacre of old tweets.

Things like greptweet.com make deleting tweets redundant though, and somebody, somewhere is making a diff of your tweets to see what you're scrubbing. Always presume you have a digital 'superfan' who never misses anything.