I thought I would share this thing that's been bugging me. You tick the "Cannot be blank" checkbox in infopath, in the browser - you get that stupid popup that gets in the way. there is a thread about it that doesn't offer great solutions, just work arounds:
http://www.infopathdev.com/forums/t/2993.aspx?PageIndex=1
I eventually worked out this solution, just modify the core.js file located here:
C:\Program Files\Common Files\Microsoft Shared\web Server Extensions\12\TEMPLATE\LAYOUTS\INC
Replace:
objVisualizationDiv.style.display = "inline";
With:
objVisualizationDiv.style.display = (strHelperName == "ErrorTip") ? "none" : "inline";