I think we can look at this again in 1.1 as I think all the mb_xxx stuff was part of PHP core starting at 5.3 so we are ensured its available .... 1.0 we still support 5.2 (made sense when we started!) although its strongly recommended to be 5.3 since that is the direction we are on with 1.1
Util also has some character encoding logic that I'm not sure we can completely drop in favor a mb_ function but then again have not looked at that file in some time. The old upper / lower case thing and that translation file should be able to go.
Character encoding is respected by mb_.
On my FreeBSD server with PHP 5.5, multibyte is still an external extension required to be built separately. Not sure how it is on other systems. emanuele kindly (and weirdly) noticed that my language file could fail (and spit out PHP Errors, I guess) when the extension isn't available. Not sure if I should add a workaround.
Strange that its external in 5.5 but hey ho thats the way it goes