It’s becoming a running joke of guessing what else will Microsoft dump from Longhorn’s first release. They started big and after several years down-played most of the highlights.
My favourite is Calibri, a lovely font for web use, which looks great in both regular and italic styles, something only Lucida Grande and Trebuchet MS does correctly now. Verdana and Tahoma are great in regular but suck in italic. I expect this font to surpass every other font now used on the Internet in a matter of few months. Corbel could have similar destiny, although is seems quite large - like Verdana - which could be problematic if you shrink it using CSS and your visitors don’t have it installed.
My other possible favourite is Consolas font, to use as editor font. I now switch between Lucida Console and Andale Mono, with the latter being my favourite, but will definitely try Consolas.
It’s a curious thing that all these new fonts start with C… Whatever happens, this will be a great contribution to web typography.