I cannot think of a single non-spam case where somebody would @-tweet the same link to a hundred people. That won't capture all spam, but it's a pretty easy low-pass filter.
True. @-tweeting the same message is indeed low hanging fruit. But spam gets sophisticated as the arms race continues.
My assertion is that after a certain point (which we are not far from), Twitter as a platform will have a problem making a distinction between "spam" and "legitimate content".
I think there's definitely potential for an arms-race here (just like there was with email spam) but I don't think it's a reason for twitter to not enter the battle at all.
Also, the @-tweeting is easy to ignore. Yes, you have to check who is replying to you a few more times than you would have, but it's not the end of Twitter.
Where you can see the real problem is when there are trending hashtags that spammers get a hold of... spammers grab hold onto a hashtag and keep it artifically trending long past its relevance.
