Another factor is that the cost/reward for mining will double (assuming same hashing power for the network) in the mid future. Quoting http://bitcoinx.com/profi

Another factor is that the cost/reward for mining will double (assuming same hashing power for the network) in the mid future. Quoting http://bitcoinx.com/profit/ http://bitcoinx.com/profit/ "When the block count will hit ~200000 some time around December 2012 the generation reward will go down to 25BTC. This might partly be compensated by falling difficulty, raising prices, higher transfer fees, etc."