I just did a little analysis of the EDU 2.0 growth rate. Below is a graph that shows growth over the 4 years that the site has been operational. I've drawn an "X" to mark the month before the start of each Fall semester.
Note there's a definite pattern. A month before the Fall semester starts, the growth accelerates and then holds steady (with predictable lulls during the vacations) for the next 12 months. Then the cycle repeats.
If this pattern continues, EDU 2.0 will pass the 300,000 member mark by the end of 2010, pass the 800,000 member mark during 2011, and pass the 1.5m mark during 2012.