Its not a 500MB cap as such - the "Unlimited" bit is that they won't automatically start charging you for going over. But they will likely contact you if you do for any significant amount. Charges will only be levied if you persistently go over, and before that they'll do stuff to your bandwidth.

There has been a ton of dissenting disucssion on the Vodafone forum, with the forum's Mods taking a bit of a beating at times. But they have all delivered a consistent message, and so I am inclined to believe it.