Please take a look at http://www.vmware.com/files/pdf/virtual_networking_concepts.pdf which explains virtual networking on ESX/ESXi. What basically happens with multiple uplinks on a vSwitch (with default settings) is that one of the uplinks will be assigned to the VM once it is powered on in a round robin manner. This way ESXi achieves load distribution across all uplinks without the need of special configurations on the physical switch ports. You can even connect the uplinks to different physical switches to increase availibility.
In case the assigned uplink fails, ESXi will failover the VM to one of the other uplinks.
André