I guarantee that Messi will not join United. Not just because the fee Barcelona will demand will be unbelievable, but also because he's been at Barcelona since he was 13, and is humble enough & loyal enough to want to stay there as long as Barcelona want him, which will be for a long while yet.
Anway, today's rumours are that we're linked with van der Vaart:
Goal.com - Euro 2008 - United, Chelsea After Van Der Vaart?
We'll have to wait until after the Euros to see if anything comes of this and the source is dubious at best, but I think if we lose Ronaldo it'd be a good move, completely different players but I expect we'll be completely changing our style anyway.