I have a RPi B+. Was running the Pre-05-05-15 version of Raspbian with all of the current upgrades/updates. Using a Wi-fi Dongle. All was well. Did a full download of the new 05-05-15 version of Raspbian onto a new micro SDHC card and reloaded everything to what I believe the original setting were. Found my wireless router and re-entered the WEP code onto the RPi. The Wi-fi symbol stopped flashing so I assumed I was connected, but was not.
The entry for the WEP key says "Pre Shared Key". I assume this is for the WEP.
I connected directly to the router via an Ethernet cable and had full Internet connectivity. Rebooted several time. Looked for new Updates/upgrades, but there were none.
Swapped back to the original SDHC card and all was well again. Everything else seems to work except for Wi-Fi.