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What are 2 antennas being used for?

What are 2 antennas being used for? WRT54G and many others have 2 or even 3 antennas. There are a number of things I could envision what they might do with this. Anyone know what they actually are doing?

Here are some possibilities.

Split band. For dual band models, one antenna could be attached to 2.4 GHz RF circuitry, while the other is attached to 5 GHz RF circuitry. This would avoid the need for an internal splitter and a dual band antenna.

Split RX/TX. One antenna could be used for receive, while the other is used for transmit. I don't see any significant advantage to this unless the TX power is low enough to still allow RX on a different channel for models than can operate on 2 channels at the same time. For remotely wired antennae, this might be an advantage.
Diversity. Two receivers work together in case one antenna position would be in a null spot for another device, the 2nd antenna might get the signal. Transmit might also select which antenna provides the better path to the destination. With extra compnents used to realize the receive advantage, this might not be done.

Directionality. Similar to diversity, but an RF stage taking signal on both antennae at the same time can get up to 3db gain in certain directions (and be entirely deaf in others). This could be used to enhance certain weaker devices and/or null out interference sources. TX could do the same. This would be difficult for the average user to manage, and require extra components, so I doubt it would be done.
Placement. Wire a remote antenna on ONE of the connectors to get coverage in two distinct places.