I don't use twitter, but what are you thinking to do with the domain? Is it possible to backup things at Twitter or ... ? Else it seems quite useless to buy it. You only have twitter as keyword in the domainname that's all.
I would stay away from any domain name using another trade name, such as "Twitter" in the actual name. A company can submit a Uniform Domain Name Dispute Resolution Policy ("UDRP") with ICANN and essentially steal this domain name from you later down the road.
Thanks for the advice, I've decided not to buy the domain. Would something like "tweet" in the domain name be under this policy?