If it were this easy for organizations to save a ton of money, it would be done all the time.
If you're just testing some things, it would probably work. Otherwise, I think MS imposes limits either in the registry or with licensing that prevents you from using it as a full-blown corporate or business web server.
I think there were concurrent connection limits when running XP as a server that were rather low; fine for a home website or a developer's testing site, but if you're using it for business it wouldn't last a day.
On top of that I'd worry about support; if you needed tech support or a consultant to come in for some reason, they're probably going to tell you that you need Windows Server in order to be compliant. Windows 7 wasn't designed to be used this way and Microsoft wants you to pay more for the server version to use it in a server role, even if Windows 7 were technically able to handle the load (which is probably is, just artificially crippled).
In the short run using Win7 for your server would be great for testing or developing. For production environments, you're asking for long term trouble (plus some software is keyed to check versions of the OS, and won't install in non-server platforms). Is it worth the potential headache? That's up to you to decide.