I've blogged earlier about IE 8 and how it might impact you as it gets out into the wild.  http://blog.sitemorse.com/2008/06/ie8-standards-compliance-issue.html and generally on the subject of browser compatibility here http://blog.sitemorse.com/sitemorse-features/code-quality/.

Now that the RC is available reality is getting closer and there's less time to get all that testing done.  Recent analysis shows IE 8 having a 1.16% share of connections which puts it well above Opera.

There are some interesting features in the RC that point the finger at looming issues of compatibility,  On the tool-bar, next to the URL box and the GO and Refresh buttons is a new button that appears when IE 8 identifies that a rendering issue has occurred.  Called Compatibility View it's a toggle button it forces a change of rendering engine from IE 8 to IE 7 to keep the page looking as it should.  The user can also add pages into a compatibility list so that IE 8 will then know in the future which rendering engine to use for that page.

There are two worrying aspects to this in my opinion. 

1. It assumes that when faced with a page that is messed up a punter will look around and find the Compatibility View button and click it rather than just abandon the site as they've always done in the past
2. If Microsoft deemed it necessary to build in this capability then they are expecting rendering issues to be a frequent occurrence rather than just an occasional nuisance

So you need to get testing with the RC version now that it's available.

As I often mention at this stage in conversations on the subject - a good starting point in this is to check your Code Quality page on you latest Sitemorse audit and clear up the errors identified and then start manually testing with IE 8 installed.