<p>An application currently in development has the requirements for using MS Word to perform spell check on certain textareas within the application.</p>

<p>I currently have Office 2007 (which is the Office 12 com objects).</p>

<p>My question is, if I add in the Office 12 objects what will happen on boxes which have Office 2003?  This is in regards to both other development boxes as well as the end users.</p>

<p>Am I correct in believing that the end result will be that the spell capabilities will not be available for those users?  And if I used an Office 11 object would that mean that the users would be unable to perform the spellchecks if they have Office 07 installed?</p>

I am guessing here, but if it is as you can't use the 2007 PIA (Primary Interop Assembly) with a 2003 installation, you could try accessing the PIA via reflection as I would guess the calls you want won't change between the two, and then it won't matter - you'll use whichever is installed.  If you are installing the PIA as well, you can then either get the user to tell you which they have, or be a little more clever and just try 2007 and if it fails, try 2003.
Like I said, I'm guessing here but it might be worth a try.
EDIT: I found this
link
about Office PIA's. This refers to Excel but actually covers Office in general. I don't envy the task you have. Looks like you'll need to detect the PIA (which may or may not be installed) and act accordingly. Sounds like a job for reflection to me.