From an informative post to the IEBlog on IE 8's layout modes and how/when they're invoked.

As you may have heard by now, Internet Explorer 8 will ship with three layout modes—Quirks, IE7 Standards, and IE8 Standards. The saying goes: "put your best face forward" and, true to this, Internet Explorer 8 will use its most standards compliant mode, IE8 Standards, as the default when encountering standards content. The behavior looks as follows:

IE8 layout modes

Page Content Declaration	Layout Mode
Known standards DOCTYPEs and unknown DOCTYPEs	IE8 Standards
Quirks mode DOCTYPEs (includes the absence of a DOCTYPE)	Quirks

To invoke IE8's IE7 Standards layout mode, it will be necessary to "opt-out" of the default layout modes (see above table) by using the following <meta> tag:
<meta http-equiv="X-UA-Compatible" content="IE=7" />

Note—The <meta> tag declaration will always override the DOCTYPE.

Learn more at my 27 March 2008 presentation on version targeting and IE 8.

• IEBlog—The Default Layout Mode
• Fix Your Site With the Right DOCTYPE!—Jeffrey Zeldman
• A List Apart No. 253—IE8 and version targeting
• MSDN article on DOCTYPEs