The WRT54G series has two antenna connectors, about one wavelength apart. Does this imply a bidirectional pattern with lobes along the line passing through the two? Is it possible to select just one antenna connector? If I put, e.g., an omnidirectional antenna on one connector and a yagi on the other, can I switch antennas remotely?

Peter, AA6FI

I am waiting for my hardware to come in but the Video show there is controls in the software based on left or right antenna.

That's correct.  The GUI does control the antenna selection.  For dual antenna devices you will be able to select Left/Right/Both.  The way Linksys and Ubiquiti treat these varies a bit.

 Linksys: Selecting both antennas makes them diversity antennas where the system picks the antenna with the strongest signal.

 Ubiquiti: Uses both antennas in an attempt to open two paths between compatible nodes.  This is how they implement their proprietary Time Division Multiple Access (TDMA) airMAX protocol, using two 20MHz channels... one vertically polarized, the other horizontally polarized.  Check out the Rocket M for more info: http://www.ubnt.com/airmax#rocketm

 Andre, K6AH

A slight correction 

The using of two streams is specified in 802.11n and is not a Ubiquiti only feature,  it applies to all 802.11n devices that support data rates MCS8 and above.  This is often refered to as MIMO (multi in multi out). When using 1 stream it is 1x2 mimo (assuming 2 antennas) ,  when using 2 streams it is 2x2 mimo,  etc.  1x2 mimo is also known as "diversity"

TDMA (time division multiple access) is indeed used by Airmax as a proprietary addition to wifi to reduce collisions, this is not used by BBHN builds and has no direct relationship to the use of multiple streams.

Suppose I have two neighboring nodes: Node A, several km away, and Node B, much closer but in a different direction. Now suppose I attach to one connector a parabolic antenna which can successfully communicate with Node A but not Node B, and attach to the other connector an omnidirectional, which can communicate with Node B but not Node A. In this scenario, will I be connected to both Node A and Node B, with the WRT54G rapidly switching to the appropriate antenna? Or will the WRT54G just lock into the antenna that gives me the stronger signal (e.g., the directional antenna pointed toward the farther node) and ignore the signal from the other neighbor, coming into the other antenna? That would be unfortunate, if the "losing" neighbor needs my node to connect to the mesh. 

Thinking about my own question, it only makes sense that I would be connected to both neighbors, with the Linksys rapidly switching antennas to maintain that communication. After all, the dual-antenna configuration was designed to support multiple wi-fi devices communicating with one router.