Antennas increase the gain of the signal. If the signal includes noise/interference, that, too will be increased.
I've not taken any measurements, although daily I use 802.11g and/or 802.11b in an office with quite a few Bluetooth devices around (headsets, phones, keyboards, mice). With no noticeable interference. [I probably should try using an antenna on a client, pointed at the office from some distance away, and see how that works with the Bluetooth gear turned on vs. turned off ...]
but i read somewhere that antennas can give worse results if they are in the presence of interference. i use a ms bluetooth mouse. as we all know, they both operate at 2.4 ghz and have been known to interfere with each other, hence frequency hopping bluetooth.
has anyone out there had problems running bluetooth devices in the proximity of external antenna wifi?