Hum, whenever I've done updates, I've got plenty of "bad signatures", but I've always been offered the option to install anyway.
Maybe you could try going to the easy urpmi
site and changing your urpmi sources to a different (maybe closer?) mirror, just incase the download was buggered for some reason?
You just follow the link, then follow the instructions, and wait. If you get an updated source that gives you an error, then just go back to easy urpmi and re do the instructions, but only for the source that gave you an error.
Sorry if this isn't much help, but it's the only thing I can think of.