Unfortunately, these things aren't really around unless you're more in broadcasting and have a lot of money...vj hardware hasn't really caught up to HD. There's stuff like this: http://www.tobyz.net/tobyzstuff/projects/dvi-mixer but it's not even released yet I beleive

You might want to post more details about what you're planning on mixing..cameras, laptops, etc and where the final output is going (back into laptop, straight to projector, etc)

The one HD mixer i've used at work was this panasonic thing with interchangeable cards..but it only really mixed hd-sdi and DVI. It was also a pretty crappy piece of equipment...needed a lot of wrestling to make things play nice with it because it was working with so many different resolutions