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.