It's likely that we (in the UK at least) will start to see stocks of the Z8 in retail channels the week beginning 25th June, although due to warehouse / delivery times etc. this may slip to the week beginning 2nd July.
Vodafone will have a slight exclusive on the phone, as the initial shipments are going to them, but O2 and SIM free handsets should be available a week or so after the initial Voda shipments.
Up to date information can be found here: http://www.motosmartphones.com/z8/availability.htm