I read that Nokia uses 2 antennas instead of one in their phones to help not loose signal when the user is holding the device; would it be possible to read the signal strength and/or quality from each antenna separatly to calculate an approximate direction to the cell tower?

First, the N900 only has 1 antenna assembly for cellular, and one for Wifi/Bluetooth -- while I haven't looked into it deeply enough to be certain each of those isn't two separate antennas, their size is much more appropriate for a single antenna on their respective frequencies. (Publicly available info should suffice to confirm one way or another, so don't take my guess as gospel.)

Second, even if you had a two-antenna system, with the received signal strength from both exposed to the user, indications would likely have more to do with how you're holding it and nearby objects than with the direction to the signal source. Would still be cool, but not very useful without a tripod or similar to mount it with minimal nearby disturbances.

N900 had the weakest signal reception of any phone I have owned, so must have half of an antenna Seriously, it was so bad, I went back to my G1 and used the N900 as a mini netbook. Since the Flash 10 fiasco, it is now a media and game system only.

On Verizon now, so no harm no foul and N900 does great for what I use it for

The Sprint CDMA cellular modems in the Amazon Kindle can get the latitude and longitude of the tower from the tower. Don't know if GSM has a similar feature.

http://www.mobileread.com/forums/showthread.php?t=92306

Netmon is a really good app for monitoring your signal

While UMTS only uses one antenna, LTE have the ability to use two or more, just like wifi-n. Basically it allows them to not only divide a signal in time, but also in space, meaning there is more capacity available (yep, my mind is blown by the concept).

Now i am unsure if user-space programs will ever get access to the needed data to do anything like what is asked in this thread, but if they do a LTE phone should in theory be able to pick up the general direction of the nearby cell towers (in much the same way that the telco can help police and rescue by triangulating a phone in their network).

A different solution would be to wire up two phones so that they can share signal data, to much the same effect (i think someone in the openmoko community did such a thing as a lab experiment to map cell towers).

That is what they call assisted gps. It allows gps to lock on faster as it do not have to waste time do a general table lookup as well as give a rough location when gps coverage is poor (if one have two or more cell towers nearby).

And yes, i have seen it in action on GSM/UMTS phones.

Yes it is, but if it could give a directional approximation of where the tower is it would be even better,

I know this forum is not too pro-iPhone, but anyway

There is an application in Cydia, its called Signal, that does this exact thing.
It show the towers around you on a google map, and shows you information from the baseband.

Here is a screenshot of how it looks:

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+