Illegal .... dont think so.,
QQ is Chinas 2:nd largest .com company and QQ is , believe it or not. The worlds largest Social IM community with somewhere up to 4-500 million users. Thats more then MSN or Yahoo for instance.
China Unicom and China Mobile both offers business and professional VOIP solutions throughout China.
The technology itself will hardly be banned in China, however western companies aiming at "stealing" marketshares from national Chinese service providers.... yes for sure, they might very well be both banned and illegalized if they pose an economic threat to the Chinese operators.