Hi all,
Just got my Pi, plugged it in with a nice 1A micro USB mobile charger & suddenly the top USB decides it has had enough!
Unplug the keyboard & mouse (that's all I have plugged in) and reconnect the mouse to the top port. No dice.
Reconnect only the keyboard to the top port. Same story.
So, I'm starting to think my USB is fried when I remember I have my Kobo ebook charger.
Enter the saviour - the standard USB wall plug with Kobo cable. Reboot & all is well in Pi-land again!
Re: USB Problems? More Power!
Glad you got your Pi working properly in the end 
What is the given rating for the Kobo charger?
Many (cheap) USB chargers give a false rating on the label. Some can supply 5V and they can supply 1A, but not both at the same time. Try taking more than, say, 0.5A and the output voltage will start to drop.
The other issue is poor quality cables. Some of them have very thin wires that can't manage the current. This isn't so important for charging your mobile phone, or ebook reader, it will just take longer, but on something like the Pi that is using it as a power supply, it can be critical.
What is the given rating for the Kobo charger?
Many (cheap) USB chargers give a false rating on the label. Some can supply 5V and they can supply 1A, but not both at the same time. Try taking more than, say, 0.5A and the output voltage will start to drop.
The other issue is poor quality cables. Some of them have very thin wires that can't manage the current. This isn't so important for charging your mobile phone, or ebook reader, it will just take longer, but on something like the Pi that is using it as a power supply, it can be critical.
Re: USB Problems? More Power!
But would a reboot with the original charger also have brought the top USB port back to life? The OP does not say whether he tried that or not.
Quis custodiet ipsos custodes?