In InfoPath form services, fields that have validation will display "Cannot be blank" message when hovered over the field. This can be annoying (very annoying:@). To disable this read this or change C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS\INC\Core.js
change line from
objVisualizationDiv.style.display = "inline";
to
objVisualizationDiv.style.display = (strHelperName == "ErrorTip") ? "none" : "inline";
or comment out
border: 1px solid #B22828; background:. . .
with in .errorDiv section of ifsmain.css file in same foler
ref: read this
Thanks,