I've done a fair amount of research into external antenna addons but have not done any physical modifications due to budget restrictions. Details of my research have been noted here. To sum it up in short, one can simply clip on the antennas to the test ports on N900's PCB.

I am not particularly knowledgeable in the field of radio frequencies and their intereferences however I have had discussions with mainly joerg_rw in regards to test cases with adding antennas to said test ports. joerg_rw pointed out a case where he had his Openmoko Freerunner with external antenna attached in order to boost the signal strength for GSM connectivity but the results came out as negative. My guess is that one definitely needs the right type of antenna for starters and possibly a directional type antenna to get the most out of the signals received.

As stated on the hardware modifications wiki for external antennas, it seems like the GSM/cellular test port is easily accessible whereas BT/WLAN as well as GPS receiver's test ports are on the obverse side of the PCB (to be frank they are close to the upper portions of N900's monitor). I have thought about drilling holes into the PCB so that I can put some wires through thus have all the antennas connectors coming out from the backside of N900, and I was informed by joerg_rw as a stupid idea :|. Unless one likes to have antennas sticking out from the sides of N900 or have cables hanging out and a couple of holes on the actual N900 case, there aren't many other potential choices if one wants to connect and use those two test ports.

There was a book called Wi-Foo which has good information about wireless penetration testing (albeit for wireless LAN in particular). The same book covers key information such as the fact that the length of the cable for antenna, the adaptors (if any attached) as well as the quality of the cable makes lots of difference with the signals. Assuming one goes out and gets 3 of these hirose u.fl to RP-SMA pigtail:

(These pigtail cables are very common, and the RP-SMA's bulkhead is not too particularly big), there will be losses due to the adapter and the length. To what extent and how much signal is lost I wouldn't know exactly. However, there are amplifiers available that allows one to attach their N900 with RP-SMA for example to and boost their signal strength. RP-SMA is a fairly common bulkhead so amplifiers, surge adapters, etc, etc are generally more abundant. The problem with amplifiers is that they can only operate with certain frequencies. Wireless LAN based amplifiers are common amongst the lot.

Just some food for thought