According to

http://en.wikipedia.org/wiki/Internet_Explorer_8

Beta 1 of IE 8 was released on 5th March 2008.
A Beta 2 targeted for all consumers is scheduled to be released in August, 2008.
The final version is scheduled to be released in 2008.

http://blogs.zdnet.com/microsoft/?p=1500

Microsoft confirms IE 8 will ship this year

Quick update from Microsoft’s Financial Analyst Meeting (FAM): Microsoft Senior Vice President of Online Services and Windows, Bill Veghte, just told attendees that Microsoft will release the final version of Internet Explorer (IE) 8 to the Web “later this year.”

Microsoft has tried its best not to provide a ship target for IE 8 — like most of its Windows client family of products. Company officials did acknowledge last month that a second public beta of IE 8 is due out in August.

Microsoft has been warning Web developers to prep for IE 8, which will be more standards-compliant, to prepare now for IE 8 by adding a new tag to their sites to keep them from breaking when viewed with IE 8.