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As mentioned earlier, whether an entityID can actually be resolved into something is generally a secondary issue. However, SAML V2.0 defines a fairly obvious way of obtaining metadata about a given entity by resolving an entityID URL (see section 4.1 of the SAML Metadata Specification).
For this reason, it can be prudent to select a URI that you control directly and could resolve at some future date. This is generally not difficult to do because a well-chosen name that has good persistence will usually correspond to a service's public/logical DNS name. When you offer a service to a significant numbers of users, getting them to switch to a different name after they're used to one is effectively impossible.
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