I know that this is quite an old thread, but this a subject which does seem to crop up on a fairly regular basis, especially as more than a few users seem to be having problems setting up USB wi-fi adapters, when plugged into their Raspberry Pi, or rather into a powered hub.
One solution which has already been mentioned is the Vonets VAP11G WIFI Bridge, obtainable from Amazon, and elsewhere, or another alternative, which I am using myself is the small TP-Link-TL R702N nano router. (http://www.tp-link.com/en/products/deta ... =TL-WR702N
This little device, which costs only a few pounds more can be 'run' in either AP, Router, Repeater, Bridge or Client mode, although in this case 'Client' mode is the one to use. It just require connecting to another Windows or Mac PC first, and the appropriate settings quickly saved to the device, following the clear instructions. (http://www.tp-link.com/en/article/?id=395
It can then simply be plugged into the Ethernet port of your RasPi, and voilà... wi-fi internet access. Although it comes with its own 3 pin PSU, it can also be powered from a 5v USB socket and has both the power and Ethernet leads included.
One advantage of using something like this is of course that if, for example, that you don't have to use a powered hub... or go through the 'hassle' of trying to make you RasPi recognise your wireless adapter!