Usually everyone in the Top 60 gets a spot, however only 4 singles players per country are allowed to compete and of course the host country gets to give out WCs. I don't remember there being any qualifying in Athens.
Some of it is based on rankings in the US and also on the feelings of the USTA and chosen Olympic team coach. Sometimes, players who have the ranking, etc get left off. Lisa Raymond was left off the 2000 team even with her great doubles results, because the Williams sisters got the singles and doubles spot. When Chris Evert was playing, she decided at the last minute to play the Olympics and to make a spot for her, doubles player Elise Bergin was dumped off the team. Nice, huh?
Re: How is decided the BEIJING 2008 entry/acceptance list? (OLYMPICS)
for Athen 2004 it was like this:
Top 48 players of the ranking 6 weeks ahead (like normal deadline for ATP Tournaments) are qualified but only 4 players for each country. If a nation has more than 4 players among the top 48 the best four shall play. The others will be replaced by the best alternates who's country has less than 4 qualified players.
there will be 2 invitation places and 14 wildcards and the ITF and the IOC will decide who will get them.
Any doubles player ranked 10 or better based on the recognized international doubles
computer ranking on 14 June 2004 will be eligible for direct acceptance into the
respective men’s and women’s doubles events. Additional pairs will be selected by the
ITF in accordance with the criteria below. There may be a maximum of two (2) pairs per
24 Direct qualifications, based on the ITF World Ranking of 14 June 2004
8 ITFplaces (Wild Cards)
Every nation can send 4 singles players and 2 doubles players. Two doubles per nation. So the singles players can also form doubles.
For Beijing, eligible players also have to have represented their countries in the International Team Championships of th Federation (basically meaning Davis Cup/Fed Cup) at least twice between 2005 and 2008, and one of the ties must be in 2007/2008, which makes the selection all the more confusing.
This is why Shazza has been so keen on showing that she's available for the Russian Fed Cup team.