For this year, the A standard is 2.11 and the B standard is 2.15. In track events, any country can take up to three athletes if they have achieved the A standard within a set time period before the Olympics, normally Jan 2007 to August 2008 in this case. Think it's the same for the marathon but in World Champs they have sometimes allowed up to 5 in the team.
So, if you got sub 2.11 in the qualification period and no-one else did, you would go. If you and five others achieved it, then the selectors would have to choose which three ran, depending on factors such as pb, championship experience, age, potential etc.