Couldn't find this anywhere else, So here goes....
The Nintendo Europe website has comfirmed a release date for Mario Party 8. It was posted on the site on the 10th May but I've only just found it lol
Anyways the release date for Europe will be 22nd June
Source10th May 2007- You’d better be in the mood for fun, as Mario Party is back! This classic party game series provides hours of fun for everyone regardless of age or gaming ability and Mario Party 8 is no exception. This fast-paced and crazy game is the first Wii instalment of the popular series and is the perfect game to play with family and friends. Join the party this summer with Mario Party 8 when it launches across Europe on 22nd June 2007.