I stared it for a good minute thinking maybe I wasn't being served the new font, but no, it's just so similar to Helvetica (Roman) that I have to deliberately look for the differences to tell. I compared Chirp and Helvetica side by side and the only significant differences in the characters themselves are in the upper case J, Q, and R, then lower case g and the numeral 2.
Am I missing something or is it just that I go to twitter so seldom that I assumed it was always Helvetica?
Edit: it seems it's not the font itself, but rather something going on with the rendering making it blurrier for some people than the previous typeface.
There's something really strange that happened to me for over a year, and it's stopped. I can't remember what it was or why. I think using https://twitter.com (without the www), forcing https, and using Safari. Some combo of those things made the site almost never load properly. Try adding in a WWW or a different browser if you're interested.
