I was searching for a long time for documentation supporting something I had learned at some point along the way: that you should only use similar NICs in a bond on ESX server. VMware's KB and Forums yielded no results. Finally I stumbled across it on page 365 of the ESX 2.5 Admin Guide:

Binding Physical Adapters

You can group physical adapters by "binding" them together. This is the functional equivalent for NIC teaming in ESX Server. Certain options you can configure through the Service Console refer to grouped adapters as a "bond".

You should bind together similar physical adapters whenever possible. ESX Server uses only features or capabilities common to all adapters when defining the functionality of a bonded switch. For example, ESX Server can use a hardware acceleration feature for a bond only if all adapters in the bond include that feature.

Hardware acceleration features supported by ESX Server include:

• VLAN tag handling
• Checksum calculations
• TCP Segmentation Offloading

Binding together identical models of physical adapters ensures that all features of the adapter can be used by ESX Server.