Jerry Coyne has discussed human brain size evolution a couple of times recently, the latest one here.
Anyway, I grabbed the data from Lee and Wolpoff (2003) ("The pattern of evolution in Pleistocene human brain size", Paleobiology, 29(2), 2003, pp. 186–196), read it into R and looked at log brain size (since linear trend on the log scale corresponds to constant percentage growth). If there was a sudden jump in growth rate, it should show as a kink.
I then used the lowess function (which is a form of locally-linear regression) in R to smooth the data, to hopefully identify any such kinks and see where they fell. I used the default value of the smoothing parameter (f = 2/3). I then tried a range of other f-values, and all values between roughly 0.5 and 0.8 (a fairly wide range, so the conclusion is robust to the smoothing parameter) give very similar-looking fits, and a clear kink at the same x-value:
The smooth shown in green here is for f = 0.8
(Click the image for a larger version)
With this analysis, the kink plainly appears at 300 thousand years ago (but the ages of the observations are approximate and subsequently rounded).
There does seem to have been a substantial acceleration in growth in brain volume approximately 300 thousand years ago.
Edit: Here's a link to a somewhat related article at Panda's Thumb from some years ago, based on a different paper. Much of the data is the same, but contains additional information.