mobile phone cards [SIMs / subscriptions] in average". (It's on page 35 of the report, or page 36 of the PDF). In other words, there are about 450m unique individuals using mobile phones in China - considerably less than the 600m suggests.
What appears to be happening is that Internet use in China is being catalysed by wider availability of broadband, and more affordable PCs. 214m out of the 253m users are broadband-based. At the same time, there is some mobile use of the Internet - 73m users of the total access on phones - but virtually all of these are PC-Internet users as well. (There's no double-counting of mobile broadband as China doesn't have 3G yet). Looking at some of the charts on the CNNIC website, it looks like China's Internet use has hit a sudden point of inflection in the past 12 months, and is now on a steepening S-curve trajectory. Mobile is still growing extremely fast, but it doesn't seem to be accelerating at the same level.
This does not necessarily mean the same trends will be seen elsewhere in the developing world. It's worth noting that China is heavily pushing the rollout of fixed broadband - something which is much slower in markets like India and most of Africa. But it does suggest that China is extremely unlikely to have a future population of mobile-only Internet users.
One other interesting snippet from the CNNIC report - the average home Internet-connected PC has 2.7 users. This is worth remembering when considering all the stats on PC vs mobile handset shipments.