I've just read this on the BBC news website
Amazon say that Vista will be available from the 30th January 2007
Prices:
Vista Home Basic: $199 (approx £106)
Upgrade: $99.95 (approx £53)
Vista Premium: $239 (approx £126)
Upgrade: $159 (approx £85)
Vista Ultimate: $399 (approx £210)
Upgrade: $259 (approx £137)
Vista Business:$299 (approx £158)
Upgrade: $199 (approx £105)