The considerations of the costs versus gains of intervention might have prevented RBI from intervening aggressively, though there has been some intervention in the market, as reported by some of the newswires.
While the current attack on the rupee is primarily sentiment-driven, on account of the growing turbulence in Europe, the rupee has developed a strong depreciation bias, due to India's widening current account deficit and growing dependence on short-term flows. These factors may not allow our currency to recover substantially (and sustainably), even after RBI intervenes aggressively. Also, according to RBI's report on forex reserves in August, India's external liabilities are more than its external assets. So, RBI would like to use forex reserves more prudently. Besides, a depreciation bias would help exports and employment generation, which is the need of the hour.
We also need to understand central banks the world over prefer selective, rather than complete, public disclosure of their interventions to make such moves more effective.