Oracle's Unbreakable Linux is not "simply" a copy of RHEL; it is compiled from the sources used by Red Hat to compile RHEL. The binaries you get from Red Hat are not identical to the binaries you get from Oracle. If you went to the auto parts dealer, bought the pieces of a Prius and then assembled it yourself, would your insurance company give you the same rate they give other Prius owners? Maybe your self-assembled car looks and acts like any other Prius, but what if . . . ?
According to Oracle's FAQ, you can use any RHEL-certified hardware or software with Unbreakable Linux, but the implication is that you will receive your support from Oracle, not from the hardware and software vendors. IBM seems to be the only company that's made any noise about not supporting their software on Oracle's Linux, but that doesn't mean other software vendors are about to do so. If a company is putting its money on the line in supporting something, they will want to do the necessary testing. It's possible that testing is going on at this very moment, and it's also possible that Oracle is paying for that testing, but we don't know.
Right now, some people are substituting the Oracle Linux product for their RHEL and reporting it works fine, but what if . . . ? I don't think it's the larger shops that are doing this; it's the smaller ones who feel that Red Hat is too expensive for what they are doing. Add to this the reports that you can painlessly substitute CentOS for RHEL, and you see a clear indication that it's the smaller shops doing switching out Red Hat.
Oracle has been working to accommodate smaller businesses and hopes to get them in the Oracle habit; that is, have them as Oracle customers. Oracle has offered a small-scale product for smaller shops for some time now. Their offer to support MySQL is likewise a play for smaller customers.
I think they see two large trends for IT besides the growing use of open source.
Questions about the very idea of Oracle offering a Red Hat-dependent distro remain, not only whether the final product is the same as RHEL, but whether you want to buy a product that is always behind the Red Hat product (Unbreakable Linux 4 vs. RHEL 5). Technically, does Oracle have the necessary Linux expertise for supporting Unbreakable Linux? Financially, will the losses in these low-priced support plans be offset by the gains in new and retained customers (especially if the products appeal to the lower end of the market)?