a) The future growth rate is assumed, that's true, but 30% is the CAGR over the past 36 months. That's why I look at the growth rate implied by the current price. Solving Ben's formula for growth given the other inputs (some assumed) indicates that the price reflects a future growth rate of 10%. There's little to suggest that the company's growth will decelerate that much.
b) If the economy ever gets back on its feet, the Fed will keep rates low for at least a year or two after that. Since I don't see the US being on the path to recovery for at least another 12 months, I'd say we have another 2-3 years of low interest rates. It will take a while for rates to go up 50%. The last time they were that high was 15 years ago.