While I don't think there are any specific laws that would prevent you from doing this, personally I would be very sketched out about using someone else's web services that have not been officially published for the world.
The reason being is not legal reasons but the fact that you are taking your eggs and putting it into a basket that can be easily taken right from under you. What happens if you spend 3-12 man-months developing an app that is based on some specific web service and then the company discovers you and changes/locks down their API? Is all that time wasted? Do you have a backup plan?
Doing this is kind of like what Boxee tried to do with Hulu. Hulu has its own site but it clearly has streaming servers for its media content. Boxee tried to create their software so it could stream directly from Hulu, but Hulu had other plans. This never went to court but since Hulu can keep changing interfaces without any notice, it simply prevented Boxee from receiving its content.
So my advice is 1) use only public APIs that are guaranteed to stay public, 2) if not public, contact the company and see if you can enter into some kind of agreement or 3) look somewhere else for what you need, perhaps another open source project