blocks will be produced at a constant rate of 1 every 10 minutes. Bitcoins are actually represented by these blocks. Right now the software does not allow people to decide which bitcoin one wants to spend. I believe that there are already a number of reasons identified on this forum why that feature would be useful. Bitcoins have a date of creation, and are therefore not all identical.

As soon as this feature is added to the bitcoin client this will allow people to trade bitcoins of a different age at a different price. In other words the geometric series: 50, 25, 12.5 BTC etc will no longer be "enforced". Bitcoins, or more clearly, bitcoin blocks with different ages will then be traded at their relative market value. So the relative value of blocks may then become a smooth function w.r.t. age.