When IE8 was released I was asked to do some evaluation of our somewhat large ActiveX application to see what issues to expect. The single largest issue was the fact that suddenly none of our modal dialogs were modal any longer! Now depending on your design, this new behaviour could be disastrous (and it would have been for us). Fortunately there is a registry setting to make IE8 work as it does in IE7 and previous versions:
TabProcGrowth = 0
The value name can be a DWORD or a string (but my example assume a DWORD).
for more info on this setting and what it does check out search msdn.com for the word TabProcGrowth.