The Viper have been out in Europe for quite some time now, happen to know a Avon rep here and being told they might see some Vipers in Sept but that's if everything goes well. Avon wants to clear out the 49/50s before they start to bring in large shipments of Vipers, so they might be hard to come by at first.
Europe seems to get all the new stuff 8 months to a year before we ever se it. Dunlop has the 208RR out but it's not avaiable to the US, go figure.
Check out http://www.fwr.co.uk they have a quick review of the Vipers, I've been trying to score a set for a while but so far no luck. I could get them thru FWR but cost to include s/h is $320 not sure I want to spend that on a single set of street tires.