Here's a handy resource that describes the compatibility issue IE8 could cause for web sites targeting IE7 and the steps they would need to take to resolve those issues.

You can find it at http://support.microsoft.com/kb/952030 

For more information about document compatibility in Internet Explorer 8, visit the following Microsoft Web site:

http://msdn2.microsoft.com/en-us/library/cc288325(VS.85).aspx

For more information about Cascading Style Sheets Compatibility in Internet Explorer, visit the Following Microsoft Web site:

http://msdn2.microsoft.com/en-us/library/cc351024(VS.85).aspx

Murray Gordon ISV Architect Evangelist Microsoft Corporation  http://blogs.msdn.com/MurrayG