I have had a lot of problems setting up this particular wifi dongle on Raspberry Pi. It is a Prolink Wireless-N Nano USB Adapter, WN2001. I like it because it is small compact, really fit to the Raspberry Pi.
When I googling I see that I am not alone. I followed all advises in many articles, including install compatible drivers, tried various setting options in /etc/network/interfaces file, etc. After many tempts I finally able to make it work.
I re-setup my rPi with latest Wheezy image from fresh. I found that the solution is very simple. I use the plain standard interfaces file coming from Wheeze, do not use wpa-roam (wpa_supplicant). In this example, I assign static IP for raspberry. Copy the below script, paste to the file and save.
sudo nano /etc/network/interfaces
iface lo inet loopback
iface eth0 inet static
iface wlan0 inet static