Twitter is one of the hottest debated topics in the web industry in general. However with more and more companies beginning to realise they need to work on SEO to maintain a web presence .... I ask the question "Is the value of a tweet (or twitter account) really going to help your SEO efforts?" As with most web companies I do have a twitter (its both personal and business), Follow Me
Twitter is not a media I would advise you to use to drive traffic to your site (though it can if you have a large enough following), leave that to DiggIt, SlashDot and StumbleUpon. So why use twitter? Twitter is at the heart of Social Media / Networking, it is about building connections and relationships, growing your network of business contacts and communicating.
Spamming twitter with links, doesn't achieve anything but you being unfollowed or not followed in the first place. Taking your time to research twitter and your niche in twitter will reap dividends. Just signing up and immediately thinking you will be successful (as with all social media) will never happen, it takes time and care.
Twitter is not an SEO tool but rather a networking tool, don't mix them up.
[Edit - 22/2/09]In addition to the above I feel I should make it clear... I LOVE TWITTER! and I follow 100% of my followers - each contact I make is important and if a contact asks a question they get priority! and they can of course get a discount if they tell me their twitter name! [end edit]