Thanks for the answer - I was afraid that there was nothing built in for this but it never hurts to ask. As I do not know much about workflow syet I think I will just post the form to an (yet to write) extension of mine which performs the required validation (Javascript is not sufficient in this case) and then does the publishing. Regarding the credit card info - it was more of an example. After reading a bit about it I know that this is nothing I would want to do. Another example for conditional validation would be e.g to force the customer to state either his phone number or his fax number - if he has given one I don't need the other.
I didn't want to scare you with the workflows, just warn you to not underestimate the debug time ;)
As for the extension, it looks like a good idea. Instead of writing a new one, I'd suggest you to have a look at powercontent from kristof. I don't think it'd be too complicated to add custom validations on his work.
If you want, contact us by email (ez at sydesy dot com ) so we can dig that a wee bit more.