IE8 is still in it's beta stage and expected to be released in this or next year. [more]
But here's an overview of what's to come : What's New in Internet Explorer 8
Or register to Microsoft Connect and access this link Internet Explorer Beta Feedback
There are a lot of links to checkout but here are some mentioned from the connect link:
• What’s new in Internet Explorer 8 for Developers
• Internet Explorer MSDN Developer Center
• Internet Explorer Team Blog