As I found here (What is Gas Limit in Ethereum?) and here (The Ethereum gas limit seems prohibitively low. Will it ever be raised? If so, when and how?), the gas limit is gradually growing (as the network grows). But is it possible that it will sometime drop below its current level or even below 3,141,592?
Here are the current values for Geth:
MinGasLimit = big.NewInt(5000) // Minimum the gas limit may ever be. GenesisGasLimit = big.NewInt(4712388) // Gas limit of the Genesis block. TargetGasLimit = new(big.Int).Set(GenesisGasLimit) // The artificial target
Miners can increase or decrease the block gas limit by a factor of 1/1024 each block, up to a minimum of 5,000. (There is no specified maximum limit.)
However, in Homestead, Geth and the other clients target a block gas limit of 4712388 (1.5 PI million). Clients that use the above values, will "vote" up the gas limit to 4712388 if it is lower.
After Ethereum launched (Frontier), the target gas limit was increased from 5,000 to 3141592 (PI million), as Vitalik Buterin explained:
The basic idea behind gas limits in ethereum is that each block can specify any gas limit within ~0.05% of the previous block, so in the long term it literally is a matter of "voting" on what the gas limit should be. Currently, all clients are set to vote 5000, but with this new update the vote will be changed to equal the "target 150% of EMA with a minimum of 3141592" policy.
In Homestead, the policy was changed to a minimum of 4712388.