I think the recent price rise can be explained by a number of things that have recently happened and it now seems to be having a simultaneous effect.
1. Berkshire will be reporting earnings soon--I guess the feeling is that the earnings will be good and if that is the case, for the P/E ratio to stay the same, the price of the stock must climb. (Very simplified explanation)
2. The silver rumor
3. The recent publicity concerning the latest Paine Webber (including NY Times coverage)
4. The knowledge of everyone that BRK is the best stock to have in a down market.
5. The fact that Buffet recently had a lot of $$$ on hand, if he feels that the stock is undervalued (as alluded to in PW report), it is possible he is buying some (which would make me very happy).
I for one also think that I faked out the stock, acted like I was going to sell it for EMC (which I almost did after heavy comptemplating). The market, sensing I had made the move, reacted by boosting the price of BRK and decreasing the price of EMC (i.e. Murphy's Law is in effect)
I would be interested in hearing other reasons (or ideas that there is no reason) behind the recent stock activity :-))