Still have users with InfoPath 2003
While InfoPath 2007 has some great new features I am sure that you have users that still running InfoPath 2007. One feature of InfoPath 2007 is the ability to save InfoPath 2007 forms backwards compatible to InfoPath 2003. To use this feature make sure that when saving the InfoPath 2007 for make sure that you select the InfoPath 2003 Form Template option as shown below.
This feature allows you to save your InfoPath 2007 form template in a format that is compatible with InfoPath 2003. You can also use this setting on a form by form basis or even make InfoPath 2003 the default format for saving any template that you design using InfoPath 2007 using the Tools – Options – Design tab.
The Fine Print
It is important to remember that if you use any new features of InfoPath 2007 these are not available and if they are part of your form you will receive an error when you try to save the template in a previous version.
One way to validate your form is to use the design checker. This is available from the design pane.