Many registries now block mixed script english/cyrillic because they can easily be used for phishing domains. Existing mixed script domains like that are now also actively blocked in Firefox etc IIRC.
Firefox doesn't block domains; it displays them in Punycode (so Wikipédia would be xn--wikipdia-f1a, for instance) if they're in some top-level domain it doesn't have whitelisted.
Presently, it seems that .org .info .biz don't support Cyrillic at all, while .com and .net don't allow Cyrillic and Western characters mixed in the same domain name. Perhaps some small country code like .cc or .ws might still allow this sort of thing, but AFAIK none of the major ccTLD's will register whateverЯus.com, whateverЯus.net and the like.
[This post has been edited by c_a_b_kingston_ca on Oct 21, 2007 10:32pm.]
Sometimes combining characters from 2 different languages into 1 IDN is not allowed. This is explained in more detail here:
Why was my international domain name (IDN) not registered?
Sometimes our domain search will say that your IDN is available, but we will not be able to register the domain for you. The problem is caused by one of the following reasons:
The IDN you entered contains characters that are not supported. The central registries do not support every character for every language. For example, in Greek there are some characters that are actually the combination of two simpler characters. The registry will support the simpler characters, but not the combined character. Another example is Verisign, the central registry for .COM, .NET, & .CC, does not allow the Euro symbol.
A variant of the IDN already exists. Some languages have two ways to write the same character. For example, Chinese has traditional characters and simplified characters. If someone has registered the traditional version of your IDN, then you are not allowed to register the simplified version.
The IDN you entered combines two different languages. Sometimes combining characters from two different languages into one IDN is not allowed and will result in a "charset problems" error in our system. For example, a domain containing Cyrillic characters can not also have English characters. This is because the Cyrillic alphabet is related to the English alphabet and combining characters from both languages could lead to confusion. Other language combinations are allowed, like Chinese and English.
Unfortunately, the IDN domain search will not detect these errors. The errors can not be detected until we try to register your IDN. If for some reason your IDN can not be registered, you will be issued an account credit which can be used towards your next Dynadot order.