There is no issue with loops, since a vSwitch connected with multiple physical uplink NICs doesn't bridge these uplink connections to each other like a normal physical switch. A (broadcast) frame will never travel through one uplink pNIC and then be sent out through the other uplink pNICs, instead it will only be forwarded to the VMs of that port group. The uplink connections are solely for the purpose bridging the VMs traffic to the outside. You can only create loops if you set up VMs with multiple vNICs connected to multiple uplinks and configure the guest OS to bridge these connections.

Check out these great whitepapers on how ESX networking works, including comparisons between ESX vSwitchs and ordinary physical switches.

http://www.vmware.com/files/pdf/vmi_cisco_network_environment.pdf

http://www.cisco.com/web/BE/learn_events/pdfs/Server_Virtualization.pdf

VMware infrastructure enforces a single-tier networking topology within the ESX Server. In other

words, there is no way to interconnect multiple virtual switches; thus, the ESX network cannot be

configured to introduce loops. Because of this, the vSwitch on the ESX host does not execute the

Spanning Tree Protocol (STP).