A handset does not require 2 antennas to monitor more than one cell, the handset maintains a list of the monitored cells in something known as the active set along with their power level, I am unsure if this information will be available through APIs to a programmer though.

Only devices using MIMO will have more than one Cellular antenna as mentioned before, however I believe that UMTS (3G) also supports MIMO but only for HSPA+ at speeds not currently implemented i.e 21MBps+

I thought i read Nokia say in a response to things Apple said duringn the antennagate fiasco that they make lots of careful studies about how people hold the devices and they also use two antennas to make sure their phones will get good signals regardless of how they are being held

only speculating but this could be based on a-gps data.

There is a program for Symbian that gives a 120 degrees accurate direction to the current tower, but its not relative to the device's orientation (you gotta have the top of the screen pointing north to have the direction be right), it's my understanding that the towers send signals with 3 individual antennas, each 120 degrees apart, and somthing in the signal identifies which of the 3 the device is connected to; i imagine that if that info was acessible for more than one tower at once it might be possible to get an approximate geographical position for the device itself (if the location of the towers is known). It's not the same as usign the difference between two antennas to try to find where the tower is, but this would also be somthing interesting to have in the N900

The WiFi/BT one is definitely shared.

It's hard to comment on this without knowing the source, but the main reason for the iPhone antenna troubles isn't the amount of antennas, but that fact that the antenna is on the outer shell where the users can touch it. In Nokia phones it is under the cover, so even if the user's hand can weaken the signal, the effect isn't as devastating as on the iPhone.