Quoting Francisco:
Besides protocol restrictions there could also be policy restrictions. For example, there could be a policy prohibiting TLD labels that have code points from Latin and Cyrillic scripts.
ICANN requires that a gTLD adopts the following policy before freeing it from a general contractual prohibition on accepting IDN registration: "All code points in a single label will be taken from the same script as determined by the Unicode Standard Annex #24: Script Names <http://www.unicode.org/reports/tr24>. Exceptions to this guideline are permissible for languages with established orthographies and conventions that require the commingled use of multiple scripts. Even in the case of this exception, visually confusable characters from different scripts will not be allowed to co-exist in a single set of permissible code points unless a corresponding policy and character table is clearly defined." Of course, if there were any language that is so regularly written with mixed Cyrillic and Latin letters that a justifiable TLD label could be proposed with them, root policies might require additional constraint. Asking again -- is there any such everyday writing system? If not, putting this on our agenda is redundant at the very least. /Cary