So ShortForm just figured this out, but here’s a pictorial explanation.
Supposedly the #NewNewTwitter has a sweet new way of embedding tweets. Finally, amirite? But here’s at least one problem.
While they look great on the front end of Tumblrs:

The backend is another, odd and confusing story:
Maybe if the Twitter user’s handle was displayed above the blockquote it’d be OK, but currently it’s difficult to tell this is even a tweet. Grrrr. Granted I haven’t gotten the New New Twitter yet, so I can’t play around with it.