The term "enterprise solution" is too vague? what does that even mean? Just saying it generically, then maybe one would say that IP03 is not generally used for enterprise use, especially with SharePoint. However, IP03 forms can be submitted to SharePoint 2007 for rich client use just like IP07. They can't be made browser-enabled when created in IP03, but they can be created in IP07 and made compatible with IP03. Now, IP03 cannot take full advantage of the things that SharePoint and IP07 have to offer, but it definitely can be used.
So, in your situation, what do you mean by "enterprise solution," and why would you be restricted to IP03? Even if the entire user base only has Office 2003 despite it being 2 versions old in a few months, you can just give IP07 to your forms developers. They can either make the forms browser-enabled, which alleviates the problem for users having Office 2003 (as long as you have MOSS Enterprise or Forms Server), or they can create them in IP07 and make them 2003-compatible IF ABSOLUTELY NECESSARY.
What do you think?