I am going to give a somewhat different answer than Gisle does above: Regarding legal requirements ask a lawyer. However otherwise, this largely supplements Gisle's post above.
The reason I say to contact a lawyer if in doubt is that copyright law varies significantly from one jurisdiction to another and in fruits-of-labor jurisdictions it isn't clear to me where the natural clear line where copyright protections end would be in a mashup. What holds in one jurisdiction might not in another, but additionally to the extent that there is a balance between users and copyright holders, this may be different from one jurisdiction to another and from one application to another within a given jurisdiction. So definitely discuss the matter with a lawyer.
The second point Gisle makes though is a good one and that is about norms. It is important to consider that coordination and cooperation can be helpful, and so doing what the community expects in a way that works with what you are doing is a good way to build some bridges which can be quite useful later on. I would urge people regardless of the law to engage in the community, and try to work out a mutually acceptable solution regardless of what the law requires. Different communities have different cultures and to the extent you can work with a community's existing culture, you may get more support, both moral and material. To the extent the community feels there is an obligation there is often a lot to be gained through fulfilling it.
A third point I would make though is that as a businessman it is not the worst idea to figure that norms are the essence of licenses. If the norms are to not worry about parts of licenses, those will be very hard to enforce (politically and possibly legally too depending on jurisdiction), and if norms are not followed even if the legal requirements are, this can result in painful consequences. So I would say if in doubt, the norms are what you follow.
