GiST for range types (was Re: Range Types - typo + NULL string constructor)

The first thing caught my eye in existing GiST code is idea of subtype_float. float8 has limited precision and can't respresent, for example, varlena values good enough. Even if we have large int8 value we can loose lower bits, but data distribution can be so that these bits are valuable. Wouldn't it better to have function like subtype_diff_float which returns difference between two values of subtype as an float? Using of such function could make penalty more sensible to even small difference between values, and accordingly more relevant.