According to the Wikipedia article on Bitcoin:
The initial bitcoin distribution is advantageous towards early-adopters. [...] However, the amount of work that must be done for one bitcoin is currently over 500,000 times more than the amount of work at which the first bitcoins were being distributed. As more people join, and also because of a reward function that halves the number of rewarded bitcoins every so many blocks, it becomes harder to generate bitcoins over time, using the same computing power.
So how is this fair? Why should someone get money just for hearing about Bitcoin before I did?