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When someone asked the MSN rep in the Search Advertising Forum if the it would be live by September he said he couldn't give a precise date - he couldn't even confirm if it would be this year.
I wouldn't hold your breath.
You can compare the difference Yahoo and msn.co.uk ads using popular keyword terms, the additional ads showing up on msn.co.uk are the invitaion only trialists.
You can sign up for AdCenter UK now even if you haven't been invited, but you won't be able to select ads to show on co.uk domains. However they do still appear to UK IP addresses, on .com domains... not that there is much mileage in that... and you won't simply be able to change/select ads to show in the UK when it goes live to everybody in August... you will need to delete ads and set them up again targeting .co.uk