After studying the IE8 Beta 1 a little, I found it surprising that sometimes the render mode that it uses for your site degrades to IE7 rendering without being completely obvious. This is interesting because, if you are not careful, it can confuse you into thinking your site renders properly in IE8 full compliance mode. Basically if you do not have a doctype defined, your page will render in the old mode, while if you do have a valid doctype defined your page will render in the new IE8 mode. Luckily there is a bookmarklet that can help you determine what render mode is being used:

Render Mode Bookmarklet

Is it degrading to IE7 or IE6? I thought that was how IE7 implemented standards vs quirks too (the presence or absence of DOCTYPE)?

The lack of a DOCTYPE caused IE to render in quirks mode--which is the behavior I'd expect.

It degrades to quirks mode, but since there wasnt a switch for render modes in IE previously, people are probably unaware that their sites are rendering in that way.