It is one of the central ideas behind bitcoin and leaves no doubt about the anonymity of the transactions.
It is a well publicised idea and it is even mentioned on the main page of the bitcoin wiki (http://bitcoin.it).
The reason people think Bitcoin is anonymous is because they think that identities cannot be linked to the addresses involved in the public transactions.
When careful enough, bitcoin can be used anonymously. But the developers do not claim that it's anonymous by default, certainly not in the current mainline client. For example, the wiki page about anonymity:
https://en.bitcoin.it/wiki/Anonymity
Anonymity is not guaranteed. There are various initiatives under way to improve this. But people should stop thinking that anonymity is the single redeeming feature of bitcoin, anyway.