Do you mean for element/attribute names?
Since CEGUI doesn't really implement its own XML parsing it would have to somehow tell each of the parser implementations (there are what... 5 or 6 of them now?) that case does or doesn't matter. I'm willing to bet you won't find support for this in *any* XML parser. Element names are rather formally defined by the W3C and case sensitivity is part of that definition. It's sort of like asking your C/C++/[your favorite language here] to not be case sensitive some of the time. Unless you're using some really obscure language, I'm fairly certain you won't find that either.
That said, I suspect you want this cause you're typing up bunches of XML for widgets/looknfeels/etc. I guess "better" tool support would be the more appropriate fix, that way you wouldn't have to type it all!
