Whether you can get a WiFi adapter to work directly plugged into a RasPi or not depends a lot upon the version of board you have, And how good your power supply is.
version 1.0: 140ma polyfuses at the USB F1 and F2. Many problems with devices that drew over 100ma. Also had problems with polyfuses that were out of spec, too high of resistance. Direct use of WiFi without modification not very successful. WiFi draw to high.
version 1.1: 0ohm resistors replaced the polyfuses. A stop gap board before the revision. I believe D14 was removed at the HDMI also. Usually successful with WiFi
version 2.0: polyfuse/resistor pads removed, Has "mounting" holes, fixed a number of problems including the 1v8 issue where the regulator in the LAN9512 was being used rather than the 1v8 on the board. This caused overheating of the LAN9512 "chip". Usually successful with WiFi.
Overall the version 2.0 boards are the most forgiving. Can be back-powered through USB ports without modification. The ver 1.1 also can be back-powered.
Usually successful with WiFi means; If the Wifi does not draw too much power it will usually work.
Remember that F3 is only 750ma. You can usually draw up to 1A against it with no major problems. You also would need to have at least a 1A power supply. The RasPi draws just under 400ma at idle with nothing attached. All of these limitations mean it is much easier to have WiFi with a powered hub. I can use a WiFi on a couple of my boards because 1 board has a 1.1A polyfuse in place of F3. The Other is Back-powered so it does not go through F3.
Raspberry Pi Projects:
Wordpress site on Raspberry Pi
Unified USB Hub and Raspberry Pi