Ask and ye shall receive. Here's a short summary of how to think about compatibility when designing form templates in InfoPath 2007.
1. InfoPath 2003 forms will work as-is in 2007
Your existing forms should work. Period. If you find something that doesn't work, please leave a comment so we can get that bug fixed.
2. InfoPath 2007 can save as InfoPath 2003 forms
Most features actually work in 2003, we just made 2007 the default to be forward-looking. If you plan to design all your forms to be compatible with InfoPath 2003, you can change the default in the Options dialog off the Tools menu:
You can also save individual forms as the file type 2003:
When you save a file for InfoPath 2003, we automatically check this box in Form Options:
So that the design checker will show you compatibility issues:
You can get more info on each issue by clicking it, and then fix it appropriately. Forms with "Messages" will successfully save for 2003, but forms with "Errors" cannot be saved for 2003 until all the errors have been fixed.
3. InfoPath 2003 forms can also work on the Forms Server
If you have the InfoPath 2003 client rolled out to your desktops, and also have the Forms Server, then you can create forms that will open in the browser for people without any copy of InfoPath, but open in the client for those with InfoPath 2003 (and 2007, see point 1 above).
First, follow the steps above to save your form to be compatible with InfoPath 2003. Then check this box in Form Options to make the form browser-compatible:
Now you can fix any remaining issues shown by the design checker, and publish the form to Forms Server the same as any other browser-enabled form.
Hope that helps,